Of late, Robert De Niro has had far more success doing comedy than the heavy drama he’s long been famous for. Though he’s suffered some stumbles (most notably his agonizing supporting turn in the lamentable live-action version of “Rocky and Bullwinkle”), his willingness to spoof his own tough-guy image in “Analyze This” and “Meet The Parents” worked well–while more serious efforts like “Ronin,” “15 Minutes” and “The Score” tanked. But with “Showtime,” a comic buddy movie in which he teams with Eddie Murphy to play a couple of very dissimilar L.A. cops who become friends while starring in a “reality” TV show, the law of diminishing returns has definitely set in; it’s nowhere near the equal of De Niro’s earlier change-of-pace pieces. The subject could have been the focus of sharp satire, but here it’s treated as bland sitcom fare instead. Though the picture gets some easy chuckles, overall it comes across as tepid, stale and endlessly predictable.

De Niro, to tell the truth, appears to be very unhappy in the role of Mitch Preston, a hard-bitten detective whose latest drug bust is compromised by patrolman Trey Sellars (Murphy), a fast-talking would-be actor Mitch instantly dislikes. When Preston manhandles a TV reporter in the aftermath of the debacle, he’s forced by his commander to cooperate with producer Chase Renzi (Rene Russo) by starring in a “Cops”-style series in which, needless to say, he’ll be partnered with the obnoxious Sellars. What passes for comedy mostly consists of decidedly forced banter between the gruff veteran and the absurdly confident wannabe, as well as Sellars’ con-man patter and the ham-fisted efforts of Renzi to transform the businesslike Mitch into someone more akin to network audiences’ expectations of a hard-driving cop. There’s also a hint of (utterly implausible) romance between Mitch and Chase (it would have been a lot more interesting if she and Trey had gotten involved, but in this as in all else, the makers are obviously playing it safe). Then there are the obligatory action elements, deriving from the duo’s determined efforts to track down a drug kingpin who’s developed some sort of dangerous super-gun. The effectiveness of this smash-and-burn part of the picture is undone by the commonplace quality of the shootouts and car chases, and the fact that the villain is a completely colorless fellow.

From all this it should be clear that virtually everything about “Showtime” seems recycled. Even the movie’s title (which it shares with Renzi’s series)–derived from the catch-phrase (“It’s showtime!”) delivered by Sellars every time he jumps into action–comes across as generic. What’s perhaps more surprising is the total lack of chemistry between the leads; they might as well have filmed their scenes separately and then just had them spliced together in the editing room. (Sophomore helmer Tom Dey had far better luck with Jackie Chan and Owen Wilson in his feature debut “Shanghai Noon.”) An aggressive, over-the-top Murphy does the loud, frantic shtick he’s mostly avoided since early in his career; there’s certainly nothing subtle or refined in his work here. De Niro is even worse. For the most part he seems completely undirected; there’s no shape or rhythm to his performance whatsoever. Some of the time he appears to be improving badly; that’s particularly true of the moments when he’s supposed to be speaking directly into Renzi’s “interview” camera (or simply glowering at it)–he’s so awful in these scenes that the sole saving grace is that at least he doesn’t mimic Travis Bickle’s “You talking to me?” routine from “Taxi Driver.” As for Russo, she’s wasted in the thankless role of a driven career woman–she pushes hard (too hard), but there’s nothing there to work with in the first place. William Shatner repeats his turn from “Miss Congeniality,” playing himself as the clueless director of the “Showtime” series; he’ll get easy laughs from viewers whose TVs are tuned permanently to TV Land for reasons of nostalgia, but basically he’s just going through the motions. Pedro Damian is a decidedly pallid bad-guy, but Brad Slocum is briefly amusing in a single scene as an unprincipled network executive. His dialogue consists of obvious stuff, but readers of magazines like “Entertainment Weekly” will probably think themselves bright for catching the references. Technically the movie isn’t much; most of the interiors look fairly cheesy, and the big action scenes (a raucous heist sequence, the big showdown in an L.A. hotel) aren’t particularly well choreographed. It just proves the old axiom that mediocrity of writing will inevitably spawn mediocrity of execution.

In sum, while a few scenes in “Showtime” are mildly funny, the picture is mostly a sad assemblage of hoary gags and obvious action effects, and the execution is surprisingly clumsy considering the cast. Like the series it so ineptly portrays, it deserves a quick cancellation.