This is a breakthrough performance for him because he creates his character in an openly, brashly entertaining way, especially when he's flirting with Madolyn (Vera Farmiga), the police department psychologist. It's such neat work because Damon is "convincing" as the mysteriously corruptible pug-nosed altar boy without taking the role more seriously than the picture warrants. He gives Collie the roundness and vulcanized bounce of a cocky-brainy guy who has never met anyone he couldn't deceive, and it doesn't matter that we don't believe in the means by which he does it — e.g., sending nick-of-time text-message warnings to Costello from a cell phone he has to keep in his pocket to avoid detection. (Surreptitious cell-phone messaging is even less photogenically suspenseful than after-hours file-cabinet rifling and race-against-time xeroxing and computer file deletion.)

In other words, Damon plays his role comically, in a slightly distanced, aestheticized manner. If he doesn't quite have the high artifice of certain commonwealth actors, from Laurence Olivier through Peter O'Toole right on up to Guy Pearce, Ewan McGregor, and Chiwetel Ejiofor, he does take as much gleam-eyed pleasure in acting here as Montgomery Clift did in Red River (1948) and The Heiress (1949), and without Clift's youthful indolence and self-regard. Damon is hard-working but all the desperation is Collie's.

Damon is lucky that the earnestness has been left to DiCaprio, who's better at it. The undercover cop's role has been expanded more from the original, though to less effect. I do believe that Scorsese, Monahan, and DiCaprio think that there is some genuine psychological probing of character going on here, i.e., the wages of doing good by doing bad. Concededly, Billy's activities are more plausible than Collie's; he does the kind of things undercover cops have to do. But psychological realism is built up from observation, and that is clearly not how The Departed was written. Moreover, to the extent DiCaprio pulls realism off, the movie becomes lopsided.

I found the claims of psychological depth easier to dismiss than DiCaprio's performance, however. He's so imaginative and physical an actor that the role of a sheep in wolf's clothing — more conceit than character — comes together. DiCaprio has beefed up and as a result has a carnal presence, and a command of means and space, unlike anything he's shown onscreen before. He can't overcome Damon's advantage in not taking the proceedings too seriously, but I can't imagine any other actor who could have made DiCaprio's choices and performed them any better.

The movie gains life entirely from the interplay among the actors, and both of the young stars are wonderful opposite Farmiga, who, with her mournful receptiveness, is possibly the least bland shrink in movie history. (No actress has ever been more memorable in the classically thankless role of the good girl observing the men in a crime picture.)