Ben Stiller has enjoyed a fruitful career playing the same character in nearly every movie.

Whether it’s “Tower Heist,” “Night at the Museum” or “Mystery Men,” he delivers a slightly agitated but well-meaning hero. A pragmatic idealist frustrated by the failings of himself and others. An underachieving overachiever.

But Stiller is quite proficient at this character. As with many of his prior efforts, in “The Watch” he provides the glue that holds all the weird and wacky happenings together.

The film is ostensibly a “sci-fi comedy thriller,” but it’s really an excuse for Stiller and his funny pals to riff off one another. The stars’ improv-heavy camaraderie works better than the flick’s more blockbustery action bits.

Stiller portrays Evan, a Costco manager in the suburban haven of Glenview, Ohio. He’s a busy guy who serves on the city council and organizes various clubs in an attempt to connect with his fellow residents. And he’s very proud of the fact that some of these people are actual minorities.

When a night watchman suffers a graphic demise at his store, Evan takes it upon himself to find the murderer and ensure the community’s safety. So he organizes a neighborhood watch (the film’s original title before the Trayvon Martin shooting in Florida compelled 20th Century Fox to alter it).

Evan hopes to recruit “a task force of concerned citizens,” yet only three oddballs respond to his flier. Bob (Vince Vaughn) is a wealthy loudmouth easily distracted by mundane trinkets and dedicated to keeping his teenage daughter chaste. Franklin (Jonah Hill) is a failed police recruit who can’t wait to commit violent crimes en route to stopping violent crimes. And recently divorced Brit Jamarcus (Richard Ayoade) displays genuine politeness to accompany his passive disconnect to reality.

Individually, they’re quirky. Together, they’re sincerely dysfunctional. Thus, when they uncover an alien invasion that threatens to destroy Glenview, it proves much more difficult to thwart.

When discovering the only clue at a victim’s murder site is a sticky green ooze, Franklin asks, “Had he just won a Nickelodeon Kids’ Choice Award?”

Director Akiva Schaffer (best known for his digital shorts on “Saturday Night Live”) is buoyed by a lively, raunchy script from writers Jared Stern, Evan Goldberg and Seth Rogen. The ratio is about 70 percent humor, 20 percent action and 10 percent horror.

The story never rises to the comedic level of “Men in Black” or “Galaxy Quest,” other enterprising pictures that blend science fiction and comedy. That’s mainly because the plot isn’t as important as just letting the leads interact. Whether sitting in a car during a stakeout or hanging around Bob’s garish man cave, the actors always capitalize on their shared chemistry.

While Stiller, Vaughn and Hill play to their customary strengths, the real casting revelation is Ayoade (pronounced “eye-oh-WA-dee”), a towering British TV comedian with a trademark fuzzy hairstyle and thick-rimmed glasses. “The Watch” offers a breakout role in much the same way “The Hangover” did for Zach Galifianakis. Ayoade’s ill-suited introductory speech at the first neighborhood watch gathering marks the movie’s comedic high point.

Inevitably, the film starts leaking mojo once the action takes precedence over the humor. “The Watch” decays into a CGI-encumbered battle that is both overly familiar and not especially thrilling. The reliable Stiller and company can be only so amusing when they’re forced to fire guns at a green screen, while visual-effects technicians fill in the blanks months later.