You expect (and half want) Al Pacino to do that hambone thing he does, but it's dispiriting that Robert De Niro has become such a grim, disgruntled actor — a buttoned-up scowler. In Righteous Kill, which pairs the two for the first time since Heat (1995), they're New York cops on the trail of a serial killer. And De Niro, who may or may not be that killer (I'm giving nothing away: It's the film's central mystery), doesn't do anything he hasn't done 100 times before. The corners of his mouth turn down in disgust, he spits out profanities like half-chewed peppercorns, and his eyes turn cold and dead. Is he working to stay in character as a Dirty Harry cop who despises criminal scum, or is he just acting out his resentment at being stuck in such a gruesomely obvious and inept piece of trash? If De Niro, with his frozen frown, is Mr. Dyspepsia, then Pacino is Mr. Peppy, grinning like a madman as he drops his dated hipsterisms (''Get it oan!''). It's not much fun to see these two reduced to Mad TV parodies of themselves. In the right movie, they'd stop coasting on their legends long enough to remind us how they became them.