Whether it precedes a biographical film or a historical drama, "based on a true story" has come to convey several, often contradictory, ideas simultaneously to wary filmgoers: The events about to transpire on screen really happened, to the very people you're about to see, at the same time, and to the same end.

Except, of course, when they didn't happen and the people didn't exist and we scrambled the time frame and changed the ending. (Hey, we said "based on.") This is our story, we're sticking to it, and we've left the fact-checking to picky historians, outraged family members, alert critics and Wikipedia.

The stakes wouldn't be so high if movies weren't so effective. Because the cinema -- with its outsize scale, sensory immersion and heightened realism -- tends to colonize our imaginations so completely, biographical and historical dramas are graded on a higher curve than any other genres.