Robert De Niro tends to look disgruntled at the best of times, never more so than when he's cast as the straight man in one of his lighter-side-of-Bobby-D. ''comedy thrillers.'' In Showtime, the disgruntlement takes on an unintended dimension. De Niro plays Mitch Preston, a hotheaded LAPD detective who is forced to star in a weekly reality-TV series, a kind of buddy version of ''Cops.'' On the show, he's paired with a hambone fellow officer, Trey Sellars (Eddie Murphy), who wants nothing more out of life than to act like he's Starsky or T.J. Hooker. When Murphy rolls off the hood of his car, action-cop-show style, or barks out a ''Freeze, police!'' cliché with both eyes on the camera, De Niro offers up one of his patented dyspeptic grimaces. Except that Murphy's moves aren't wild or exaggerated enough to actually be, you know, funny (he just looks like a TV-cop cipher), and so De Niro seems to be reacting to nothing so much as the lame movie he's stuck in.
Unlike ''Analyze This'' or ''Meet the Parents,'' ''Showtime'' is a lead-balloon caper, a showcase for the duller side of Bobby D. Murphy, by contrast, might almost be doing a replicant version of his live-wire younger self. His verbal energy, even at its most superficially aggressive, now comes across as an overeagerness to please. The premise of ''Showtime'' makes it sound like a media satire, but it's one of the film's more inept oddities that the De Niro-Murphy reality show, as it looks on television, remains almost entirely out of our view, adding to the feeling that the jokes in this movie, if there ever were any, must now be on the cutting-room floor.