As Dennis, the paunchy, lackadaisical everyman who attempts to recapture the heart of his estranged fiancee in “Run, Fatboy, Run,” Simon Pegg is dim, boorish and astonishingly irresponsible.

Whether he’s chasing down a cross-dressing thief or taking his son (Matthew Fenton) to a theatrical production of “Lord of the Rings,” his ability to turn simple tasks into embarrassing debacles is uncanny.

And yet he is not without charm. Selfish and emotionally stunted though he may be, he is also unexpectedly clever (when he needs to be) and endearingly well-intentioned.

Pegg has played this sort of character before, most memorably in 2004’s “Shaun of the Dead,” and it is a role he embraces with disarming humility.

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There is no swagger in Dennis, an ambitionless security guard haunted by his decision to abandon his pregnant girlfriend Libby (Thandie Newton) at the altar, and that is key to his appeal. Even at his lowest, he invites sympathy, often from those most affected by his lamentable behavior.

“Run, Fatboy, Run” is neither as funny nor as wickedly subversive as “Shaun,” but it respects its characters enough to make them thoughtful and surprisingly complex, unlike the sitcom stereotypes inhabiting lesser romantic comedies.

There’s little tension in the rivalry between Dennis and Whit (Hank Azaria), the fitness-obsessed Mr. Right who threatens to steal Libby’s heart — the outcome is inevitable, though no less satisfying for it — but Pegg and Azaria, who is deliciously condescending as a hotshot lawyer with an inflated sense of self-worth, prove fierce adversaries.

“Run, Fatboy, Run” has been derided in some circles for its crudeness — one scene, involving the popping of a pus-filled blister, is particularly cringe-worthy — but the film, which marks David Schwimmer’s directorial debut, provides a pleasing mix of the sweet and the scatological.

The story, originally conceived by Michael Ian Black of MTV’s “The State” and doctored later by Pegg, is more sentimental and unapologetically crowd-pleasing than ironic, but by the time “Run, Fatboy, Run” hits its stride, its mix of mild raunch and screwball slapstick is uproarious.

For Pegg, an unconventional leading man whose collaborations with director Edgar Wright (“Shaun of the Dead,” “Hot Fuzz”) have yielded two of the sharper comedies in recent years, “Run, Fatboy, Run” is a showcase for his winning personality and quick-witted dialogue.

For Schwimmer, it is a promising debut, a nicely crafted underdog tale that crosses the finish line with a flourish.

CREDITS

Run, Fatboy, Run (3 stars)

Starring Simon Pegg, Thandie Newton, Hank Azaria, Dylan Moran, Harish Patel

Written by Michael Ian Black and Simon Pegg

Directed by David Schwimmer

Rated PG-13

Running time 1 hour, 40 minutes