The Thin Man, based on the 1933 Dashiell Hammett novel, is overall the best and most crisply made picture in a series that endured for more than a decade. It’s most certainly the one that hangs together best as a whodunit. But then, Powell and Loy saw to it that nobody ever really cared about the Thin Man movies’ plots: Audiences came for Nick and Nora, to get a glimpse into this partnership between two stylish, quick-witted equals, to enter a world in which married life looked more like fun than like drudgery. But the true secret to their appeal may be that their dialogue isn’t just banter (face it, anybody can engage in banter): It’s honest-to-God flirtation—often of a prickly sort, but flirtation nonetheless.

This wasn’t just something new; the sight of married people flirting with one another, in the movies or in real life, is still a novelty. And as the series progresses—the other pictures are After the Thin Man (1936), Another Thin Man (1939), Shadow of the Thin Man (1941), The Thin Man Goes Home (1945) and Song of the Thin Man (1947)—we get a sense of how the kinship between Nick and Nora toughens and deepens, but also of the ways in which the two always remain like newlyweds who genuinely enjoy each other’s company. The junky plots orchestrated around Nick and Nora don’t matter: We have eyes only for them.

There’s no possessiveness in the Nick and Nora equation. Their relationship is predicated on the acknowledgment that each had a life before marriage—Nick as a playboy detective, Nora as a freewheeling, freethinking heiress—and the success of their union depends on preserving their individuality, not muting it. This is happily-ever-after as a beginning: In fact, studio heads at MGM were nervous about The Thin Man because they weren’t sure that a romantic comedy featuring an already-married couple would fly. What they didn’t realize—even though Powell and Loy, married to each other only onscreen, seemed to recognize it in their bones—is that a married couple who actually like each other is the sexiest thing in the world.

Nick flirts openly in front of Nora; she responds either by ignoring him or laughing in his face. So what if his eyes wander? She knows his heart belongs to her. Likewise, Nick can’t turn his back for a moment without finding his charismatically charming wife surrounded by potential suitors. In Another Thin Man, he disperses one of these masculine gaggles with a single line, delivered with an eloquently raised eyebrow. Professing surprise at seeing his wife out of bed, he exclaims, “What if the doctors find out? They’ll put you right back in quarantine.” Nora plays along, more interested in running with the joke than she is in these male admirers who are nowhere near as much fun as her own husband.

And we know a lot about what Nick and Nora are like in bed, even though we see them in bed—separate beds, actually—only fleetingly. One of the loveliest things about the Thin Man movies, taken as a whole, with all of their pleasures and flaws, is that Nick and Nora aren’t newlyweds, in the strict sense of that word, forever: They have a child together. (In the last picture in the franchise, Song of the Thin Man, he’s played by the very young Dean Stockwell.) In Shadow of the Thin Man, Nick urges Nora to stay out of the way while he investigates a case (an entreaty she ignores, as always): He tells her to go home, put some cold cream on her lovely face and slip into an exciting negligee—“and I’ll see you at breakfast,” he adds with an air of disappointment. In Song of the Thin Man, Nora sits at her dressing table and asks Nick to pick out a dress for her to wear that day. He holds up a filmy thing, declaring it his favorite. She tells him it’s a nightgown. “It’s still my favorite,” he says without missing a beat. The suggestion is that Nick and Nora have always had, and continue to have, a firecracker of a sex life, one that may have changed over the years, but that has never withered or soured.

In a scene in The Thin Man Goes Home (the best of the sequels), Nick and Nora’s intimate life is alluded to in a moment that’s both breezy and thrillingly erotic: The two are visiting Nick’s parents in the small town he left years ago. Nick lounges in a hammock; Nora has tried to set up a lawn chair nearby, but it keeps collapsing, ultimately with her in it. The two fall together on the lawn, laughing, and Nick leans over to kiss and nuzzle Nora, covering her body with his own.

It’s a moment that’s at once hilarious and electric; it’s also interrupted by Nick’s elderly mom (played by Lucile Watson), who appears at the door, calling sharply, “Children!” The grand but subtle joke that wends through every movie in the Thin Man series is that as sophisticated and worldly as Nick and Nora are, nothing delights them more than the act of discovering each other again and again. As the series progresses, we watch as they grow older, though no less attractive: Nick gets a tad thicker through the waist, but he always sports that mustache groomed into elegant little points, like the peaked roof of a pagoda. Nora may wear a few more practical, mom-like tailored dresses, but she never loses her taste for silky dressing gowns. They’re the grownups we all want to be, without being anything so boring as adult.

The Complete Thin Man Collection. (Warner Home Video), $59.92.

It's been about five years now since critics and audiences started sounding the alarm bells over Mr. De Niro, who, as he rounded the curve toward age 60, began taking more comedy and light character roles. Analyze This (1999) and Meet the Parents (2000) were big hits-big enough to spawn sequels, albeit lousy ones. (And dopey or not, Meet the Fockers is turning out to be a big moneymaker as well.) While critics and audiences alike have enjoyed those movies, there has also been plenty of audible grumbling that Mr. De Niro, by appearing in such trifles, is squandering the great promise of his youth.

This is, after all, an actor so great he's been mimicked at dinner parties across the land ("You talkin' to me?"). How could he stoop to making Showtime with the likes of Eddie Murphy? Supplying the voice of a mob-boss shark in Shark Tale and playing Fearless Leader in The Adventures of Rocky and Bullwinkle may have been fun, but enough is enough. Isn't it time for Mr. De Niro to get down to the real business of acting?

And that line of thinking leads to a dangerous question: Just what is the business of acting?

Mr. De Niro's résumé is hefty: He's made more than 25 movies in the past 10 years alone. There's no way for anyone but a mind reader to know, with absolute certainty, which movies Mr. De Niro has made for love, which for money and which he has made for both. Although we often assume, wrongly, that successful or much-lauded movie actors are rich, Mr. De Niro is involved with enough other projects, from directing to running his own production company (Tribeca Films), that desperation for money isn't a factor in what roles he takes.

At the same time, work is work: Any actor, at any level of stardom, should have the right to take on work that he might enjoy doing and make some money at the same time. (And as far as Fockers goes, ascribing Mr. De Niro's motivations to sheer greed is presumptuous: If he had a good time working on Meet the Parents, he'd have good enough reason right there to take on the sequel.) Whatever we presuppose about Mr. De Niro's motivations, this is the résumé of a man who, for whatever reason (and money isn't necessarily a bad one), likes to work. It's a journeyman actor's résumé, not the kind that, in our fantasies, we'd fashion for one of the greatest actors of a generation. And for anyone who loves, or claims to love, actors, it's a cautionary tale about the dangers of reading a performer's career movie by movie instead of moment by moment.

But first, let's get back to the obvious: Mr. De Niro is lousy in Meet the Fockers. The performance feels gnarled and stiff and mechanized; it lacks the jovial menace of the one he gave in Meet the Parents. In both movies, Mr. De Niro plays a parody of the unnerving, almost-ready-to-blow characters he built his early career on-the gag is that he's a suburban dad who both loves his family and clings to the idea of order. In the first picture, Mr. De Niro is clearly relishing the joke-even in the context of his character's obsessive fastidiousness, he's loose and relaxed. But in Fockers, the central gag of the character feels too internalized; it's as if Mr. De Niro, having already developed the character as far as he could, realized there was nowhere further to go with him and simply decided to coast. (The cloddishly desperate jokes written for him didn't help.)

Fockers should have been better, and Mr. De Niro better in it, but it's not the blight on Mr. De Niro's career that some longtime fans seem to think. Toting up the performances he has given in the past 15 years or so, a number of them seem vague and indistinct. You may barely be able to picture the types of characters he played in Cop Land, The Score or Marvin's Room. Those performances may have been perfectly serviceable, but they just haven't stuck.

Why has Mr. De Niro taken so many roles, instead of choosing just a few juicy ones? The answer, whatever it is, suggests that when you're an actor with a reputation to withhold, there's a danger in liking your work so much that you actually want to keep doing it. Mr. De Niro's career is peppered with superrefined, overcooked performances that have been universally accepted as great (as in Raging Bull), and great, fairly recent performances that have been almost completely overlooked (as in Alfonso Cuarón's viciously and unfairly maligned Great Expectations). Scattered across the vast plain of Mr. De Niro's career are performances that fold under their own weight (as the evil stepdad in This Boy's Life, his singsong thuggishness feels cartoonish), and performances so ardently delicate and complex that no actor since has been able to touch them (most notably, his turn as the young Vito Corleone in The Godfather, Part II).

You can dislike some or even many of Mr. De Niro's wide-ranging performances, and even feel that many of the choices are beneath an actor with his capabilities. But particularly considering that many actors work less as they get older (and actresses, of course, have it even harder), the arc of Mr. De Niro's résumé suggests not a lack of discrimination, but a total devotion to his craft-and, maybe even more significantly, an ingrained sense that in order to be a working actor, an actor has to, well, work.

Few would ever accuse Mr. De Niro of being lazy, and the first piece of evidence that's generally served up is the more than 50 pounds he gained to play Jake La Motta. But although the performance is often cited as a great Method performance, it is, as the critic Steve Vineberg pointed out in his fine survey Method Actors: Three Generations of an American Acting Style, a betrayal of Method technique: "When … Robert De Niro put on fifty pounds to play boxer Jake La Motta in Scorsese's Raging Bull, literally making himself into what earlier actors had simulated by the use of creative imagination, he was confusing actual physical transformation with acting."

You can see Mr. De Niro working every second in Raging Bull, which is precisely the problem: It's not just a textbook performance but a performance textbook. His Jake La Motta is a slab of living beef with two holes for eyes-there's humanity in there somewhere, submerged beneath all that festering, muted rage and crippling paranoia, but it's impossible to tell how either Mr. De Niro, as the creator of the performance, or Martin Scorsese, as the director of the work, feel about the character of Jake La Motta. Mr. De Niro's gears are working at full-tilt, but the overall effect is one of overactive blankness.

It's heresy to say that I prefer Mr. De Niro in some of the lighter performances he started giving in the late '90s, in pictures like Analyze This and Meet the Parents. But I'll say it anyway. Those performances shook something loose in Mr. De Niro, both as an actor and as, simply, an onscreen presence. He was able to unleash his mischievousness without ever dissolving into unadulterated, sickening cuddliness. These are performances that don't negate the terrifying and agonizingly vulnerable, unreachable specter of Travis Bickle, but flicker around it. There's no direct link between the characters of Bickle and of Paul Vitti (the tough-guy mobster who, in Analyze This, seeks the help of analyst Billy Crystal to quell his panic attacks), other than some spark of ignition in Mr. De Niro-something expressively alive and unique to him. But then, that's enough. You wouldn't view Mr. De Niro's turn in Analyze This as strictly Method. But in some ways, the performance suggests the full range of what it means to be a Method actor, if only in that modest performances can contain the glimmers of great ones (and vice versa), stretching across a range of performances decades apart.

Bad performances are as essential to a great actor's résumé as good ones, because with inconsistency comes surprise-the ace up an actor's sleeve. I feel no heat coming off Mr. De Niro in Goodfellas, and I howl when, in This Boy's Life, I hear him clatter through a line like "I know a thingger two about a thingger two." But two movies later, or 10, he'll redeem himself in an instant, as in the otherwise tiresome 15 Minutes, when he proposes to Melina Kanakaredes by blurting out, "I want to have some shoes next to my shoes"-the words are inelegant, but the rhythm is like music, John Donne reinvented by Howlin' Wolf.

Line readings are part of the actor's technique, and they're what we cling to most tenaciously when we recall a performance. But Mr. De Niro's career is more aptly defined by what he says when he's not talking. Collectively, when we think of Mr. De Niro, we picture him as Travis Bickle in front of the mirror, or as Jake La Motta beating the dickens out of his brother, and the words that accompanied those moments appear before us as if encased in thought balloons. But there are no words, and no convenient thought balloons, to go along with what may be Mr. De Niro's single greatest moment (if there has to be only one). If it's the small moments that define true greatness in an actor, let's choose the one where, in The Godfather, Part II, Mr. De Niro's young Vito Corleone returns home to his wife and young son after losing his job. He hangs up his jacket and unwraps a pear he's drawn from his pocket, which he places on the table. His wife will exclaim over it when she sees it there.

But in the moment before she does, a moment in which we see only Mr. De Niro's back, we know everything he's thinking-his feelings aren't safely encased in thought balloons; they're a wild, mournful current running down his spine.

It's a small moment, a marvel of economy, and one that dissolves the space between actor and audience until there's nothing left between us but empathy. It's as great as anything I've ever seen an actor do. It is defiantly not a line that can be mimicked at dinner parties, and it stands as a slippery symbol of the difference between everything Mr. De Niro the great actor stands for, and everything we may wish he would stand for, if only he'd stay put on the pedestal we've built for him. Confounding us, frustrating us, and sometimes even enraging us with his choices may be the only way he can command and maintain our respect.

Mr. De Niro, at this point in his career, is technically more than an actor, but none of us can pretend that it's as a businessman and entrepreneur that we care about him. Those sidelines belong to him solely, but his performances are a gift to us, and it's stingy to suggest that the lackluster ones are enough to constitute a betrayal of his original promise. The only role worse than Greatest Actor of His Generation may be National Treasure, and subconsciously or otherwise, it's one Mr. De Niro has managed to avoid. When it comes to his performances, maybe he only ever wanted to be thought of as an actor.