Comedy, more than any other genre, is like a friend. A great comedy makes you laugh a lot and never overstays its welcome. Any great friend should do the same, as well as occasionally pick up the tab for dinner. On the other hand, we’ve all had friends who just aren’t funny - they may be good people, but every joke they tell falls flat. When comedy fails, it’s often painful bear witness to, and if you will allow me to carry this metaphor to its extreme conclusion, David Schwimmer’s Run Fatboy Run would probably be a manic depressive friend.

I'm thinking they needed to find a fatter actor to play the fatboy.

The plot is a trifle of a thing. Five years ago perennial loser Dennis Doyle (Simon Pegg) left his beautiful, pregnant fiancee Libby (Thandie Newton) at the altar. Libby is now engaged to nice guy Whit (Hank Azaria), and when Whit mentions that he is running a marathon Dennis takes it as a challenge and decides to run as well. Dennis somehow thinks that by running 26.2 miles he will prove to Libby that he’s no longer an irresponsible dolt. Just to be clear, he is both irresponsible and doltish.

The surprising thing about the movie is that it manages to make us genuinely like Dennis. This is quite a feat considering that our relationship with him begins with him fleeing the woman carrying his child. His best friend Gordon, rakishly portrayed by Dylan Moran, is a drunken, womanizing gambler. Dennis can’t manage to pay his rent and he’s regularly late to pick up his son for visits, yet we want him to grow and achieve. This is largely due to Simon Pegg, who has a knack for making idiots appear adorable.

They make a cute couple, don't they?

The first half of the film is very funny, often darkly so. Pegg and Azaria make fantastic rivals, trading barbs and backhanded comments, while never crossing over into outright meanness. Much to the film’s credit, even though we know Whit will reveal his evil side in time, they do give him quite a while to be the Good Guy. He bonds with Jake (Matthew Fenton), Dennis and Libby’s son, and he seems to be genuinely interested in helping Dennis become a better man. That is, until the final reel, when he is revealed to be a full-on mustache twirling villain. The problem is that the earlier scenes with Whit and Dennis trying to balance their hatred for one another against their mutual love for Libby are much funnier.

Dennis and Whit’s relationship is indicative of the film as a whole. When writers Michael Ian Black and Simon Pegg are playing against convention they are able to mine the material for some truly uncomfortably funny moments; it’s when they start playing by the rules that the movie becomes boring and trite. I’m not sure whether to blame the writers, the director or that old stand-by ’studio interference,’ but the last half of the film is a marked departure from the first. It plays like a Disney movie-of-the-week extolling the virtues of believing in yourself.

Ultimately, Pegg is likable enough to warrant a viewing, although it would have been interesting to see what they could have made of the film without the schmaltzy ending.