I think Simon Pegg had people fooled for a while there. Because the British comic (and his collaborators Edgar Wright and Nick Frost) had found a niche in genre send-ups – science-fiction in the British TV series Spaced, zombie horror in Shaun of the Dead, American action movies in Hot Fuzz – it was easy to think of him as a descendent of Monty Python. But here’s the reality: Simon Pegg is a softie.

You might have to squint a bit to see it through the twisted moments of violence in Shaun and Fuzz, but it’s there. Both films were anchored in genuinely touching buddy relationships. And with his contribution to Run, Fat Boy, Run, it becomes even more evident. This guy is less John Cleese than he is James L. Brooks.

Unfortunately, Run, Fat Boy, Run marks the first time that Pegg’s sentimentality has gotten in the way of his sense of humor. He stars as Dennis Doyle, a somewhat hapless Londoner whose life has been defined by a bad choice he made five years ago: escaping out a window on the day he was supposed to marry his pregnant girlfriend Libby (Thandie Newton). Now a weekend dad and debt-ridden retail security guard, his world is shaken up when Libby becomes involved with slick American stock broker Whit (Hank Azaria). Determined to prove he isn’t completely worthless, Dennis vows that he’ll run in the same marathon as Whit in less than a month – which may require him to stop smoking, lose a little weight, and be able to jog more than 100 yards without gasping for breath.

If you take a look at the pedigree of Run, Fat Boy, Run, it’s easy to see how a clash of comedic styles might have led to disaster. Pegg re-wrote a script by Michael Ian Black (Stella), whose style tends toward the cerebral and surreal; director David Schwimmer has several sit-com credits, but makes his big-screen debut here. Imagine the prospect of a Friends/Shaun of the Dead/Stella mash-up, and let your mouth gape at the potential horror.

But the film does find a sense of humor, and for the most part its Pegg’s. He once again allows himself to be the butt of plenty of the jokes, and generously passes on plenty of great character moments to Dylan Moran (as Dennis’ ne’er-do-well best pal) and Harish Patel (as Dennis’ landlord). The physical humor swings to the over-the-top, including a burst blister that inspires the best single line of dialogue. Long-time Simpsons voice Azaria still seems uncomfortable whenever he’s playing a role in his own voice, but there are plenty of other funny things going on around him to make up for how straight he plays Dennis’ rival.

Perhaps not as many funny things as there might have been, though – or, more to the point, perhaps the serious stuff isn’t strong enough to justify passing on more comedy. Pegg wants to make Run, Fat Boy, Run a quest for Dennis’ dignity, and tries to make him less of a jerk by turning his abandonment of Libby at the altar into an act of misguided altruism – he wasn’t scared, he was saving her from a lifetime with a loser like him. But the film doesn’t show us enough of Dennis’ and Libby’s pre-engagement relationship to support that assertion. From everything we see, Dennis is simply irresponsible, and Pegg’s forced attempt at making him more sympathetic feels artificial. And frankly, Newton’s a bit too chilly for it to feel like getting back together with her is much of a payoff for his efforts.

Because Pegg himself is such an endearing performer – and because he and Moran share so many snappy scenes – Run, Fat Boy, Run doesn’t feel like a failure. It does, however, feel like a disappointment. Pegg and his cohorts, in their previous collaborations, had managed to combine more big belly laughs with their big hearts. Maybe Wright should be given credit for more of the punch lines. Or maybe Pegg just likes taking things in a kinder, gentler direction.

RUN, FAT BOY, RUN ( 2 ½ ) Directed by David Schwimmer • Starring Simon Pegg, Thandie Newton and Hank Azaria • PG-13, 100 min • At Century Cinemas Del Monte Center, Maya Cinemas, Northridge Cinemas.