At some point, “Showtime” stops being a parody of generic buddy-cop movies and starts being a generic buddy-cop movie. Once things get difficult, it retreats hastily to the safety of familiarity.

While the good times last, they are good indeed, and the film is even entertaining (if forgettable) when it’s being lazy. It’s about an L.A. cop named Trey (Eddie Murphy) who wants desperately to be an actor. It’s also about an L.A. police detective named Mitch (Robert DeNiro) who wants nothing to do with the limelight. TV producer Chase Renzi (Rene Russo) wants to create a reality series in which a camera crew follows Mitch around and catches him being surly and angry — he is the L.A. version of “NYPD Blue’s” Sipowicz — and thanks to some particularly volatile behavior from Mitch, he either has to agree to be filmed or else get sued.

Naturally, the most interesting partner for him on his new reality show would be someone completely opposite, and so Trey gets his 15 minutes of fame. The case they’re working has to do with an especially deadly new gun being passed around on the black market, but that sort of detail couldn’t be more irrelevant.

What’s fun, allegedly, is the opposites-attract interaction between Trey and Mitch, except that we’ve seen this exact situation before. So what’s fun is how they’re mocking that kind of set-up, except they don’t mock it enough. “Showtime” is too much of one thing and not enough of the other.

But there are high points. William Shatner, who is wise enough now to realize his best work will be in parodying himself, plays himself as the director of the show, giving Mitch and Trey acting tips. There is a world of subtextual comedy in hearing Shatner say, in regards to DeNiro’s character, “This guy is a terrible actor.”

And admit it, the casting couldn’t be better. Eddie Murphy invented the fast-talking cool black guy persona that has since been abused by the likes of Chris Tucker and Martin Lawrence; casting anyone other than him in the role of Trey would have been a crime. Ditto for DeNiro as the tough cop.

There is no particular reason to see this film, but there’s no reason to avoid it, either. It provides a few laughs and a bit of worthless amusement — not exactly a ringing endorsement, but fairly charitable, given how badly it squanders its opportunities for greatness.

C+ (; PG-13, a lot of profanity, a lot of shooting violence.)