No one’s playing to type in “Run Fat Boy Run,” the unabashedly old-fashioned, good-natured, underdog comedy set to open on movie screens next Friday. It’s not the sort of low-key chuckler we’re used to seeing compete against today’s loud and crass gross-out comedies. For that matter, it’s hardly the restrained humor one would expect from a cast that includes the co-creator of the outrageous zombie comedy “Shaun of the Dead,” a co-star of the wisecracking television institution “The Simpsons,” and a director who became famous by zinging punch lines for 10 seasons on the hit NBC series “Friends.” Two of these three personalities are already familiar, if not household names, to American audiences. Hank Azaria is the actor behind such iconic “Simpsons” voices as bartender Moe, Kwik-E-Mart entrepreneur Apu, and the arrogant Comic Book Guy (as well as many others). And “Run Fat Boy Run” director David Schwimmer is known to millions as Ross, or rather Dr. Ross Geller, the character he sustained on “Friends” for almost 240 episodes.

But while Messrs. Azaria and Schwimmer enjoy a casual celebrity on these shores, their “Fat Boy” colleague and hero, the hit British comedian Simon Pegg, is poised to use next week’s debut as a proper introduction to the American mainstream.

The 37-year-old Mr. Pegg, whose ginger hair and mild mannerisms lend him a natural everyman presence, rose to acclaim on British television nearly a decade ago with the short-lived domestic comedy series “Spaced,” and became a cult presence on both sides of the pond with his first hit film, 2004’s “Shaun of the Dead.” A genre mishmash that blended the emotional core of a romance with the bloody terror of a zombie thriller, the film centered around Mr. Pegg’s title character, a lovelorn young man who mopes around his sleepy English hamlet mourning the loss of a girlfriend as zombies in the background all but burn the village to the ground.

While American critics and fans of British comedy treated “Shaun of the Dead” as something of a guilty pleasure, they didn’t hesitate to rally loudly and publicly around Mr. Pegg’s next project, last year’s “Hot Fuzz,” which received rave reviews and earned a modest $23 million at the box office. “Hot Fuzz” took another swipe at the genre paradigm, poking fun at Hollywood’s bombastic, hyper-edited action spectaculars, with Mr. Pegg in the role of a fiery police sergeant reassigned to a sleepy suburb. Whether investigating graffiti vandals or filling out his daily paperwork, Mr. Pegg acted the part of a gritty action star, and with every click of his pen, the movie did its best impression of a Michael Bay montage.

Some referred to “Hot Fuzz” as a “blockbuster in training,” and the label could not have been more apt for Mr. Pegg’s career trajectory. On the heels of his “Fuzz” fame, the actor was tapped to replace the late James Doohan as Scotty in J.J. Abrams’s forthcoming “Star Trek” film, which is currently in production and slated to hit theaters next summer. But before that bit of science fiction fun arrives, Mr. Pegg will run onto screens next Friday not so much as a goof but as a fun and flabby everyman trying to win back his onetime fiancée by proving he can outrun her new lover (Mr. Azaria) in the London marathon. In a performance that is much softer, sweeter, and sensitive than fans of “Hot Fuzz” will be expecting, Mr. Pegg establishes himself as a most unlikely leading man — a chipper, suave, but woefully flawed charmer.

Mr. Pegg’s “Fat Boy” co-stars said the most challenging aspect of the production was finding the right balance for his silly-serious oaf, tempering the star’s comedic sensibilities, and at times convincing the comedian to abandon that mind-set altogether.

“So many people know Simon from his other movies, his very physical comedies,” Thandie Newton, who plays Mr. Pegg’s ex-fiancée, said. “The key here, though, was that we wanted the comedy to come from the reality of the situation, so David would sometimes have to remind him to underplay the moment, to not go for the joke. It was hard for me, too, because I was supposed to be frustrated with the guy, but he’s so endearing.”

For such a no-holds-barred performer as Mr. Pegg — who regularly vacillated in “Shaun of the Dead” between puppy-dog-sad and ruthless zombie killer — “Run, Fat Boy, Run” presented the curious challenge of “less is more.” Ms. Newton said that the almost schizophrenic quality that she so adores in Mr. Pegg — “his ability to go so quickly from madcap to concentrated comedy to emotional earnestness” — is still front-and-center as he jumps among the various sublots of a romance, a buddy comedy, and an inspirational sports film.

“Run Fat Boy Run” is a multilayered story that Mr. Pegg helped to write, along with the American comedian Michael Ian Black, perhaps as a means of showing audiences his softer side. Dylan Moran (“Tristram Shandy,” “Notting Hill”), who in “Fat Boy” plays Mr. Pegg’s best friend, said that the star is not only creative, but collaborative in a way that few big-screen funny men are. “Comedians are notoriously competitive, but there was a prevailing air of generosity and keenness to make the stuff work,” Mr. Moran said. “I did the picture because the script made me laugh.”

Laughing, though, and getting others to laugh, have never been a problem for Mr. Pegg, just as it’s never been a problem for Messrs. Azaria or Schwimmer. But “Run Fat Boy Run” seeks to use those laughs toward loftier, more heartfelt ends. There aren’t just punch lines in this movie, but scenes of struggles and sadness, of revelry and redemption. It’s not the movie that fans of these pranksters will be expecting, and all three are banking on that very idea.

ssnyder@nysun.com