Most movies nowadays depend on special effects to hold the audience's attention. Stranger Than Fiction has very few special effects. Your attention is held by something else: the writing.
The plot is fairly simple. IRS agent Harold Crick (Will Ferrell) starts hearing a woman's voice correctly narrating aspects of his life. He quickly comes to realize he's a character in someone's fictional story. And the author's going to kill him.
The narration is exquisitely written. The dialogue is sharp and often hilarious. There's no slapstick (which is why I normally avoid Will Ferrell movies). This is verbal humour a la British comedy. And it makes for a great film.
My favourite part is when Harold visits literature professor Jules Hilbert (Dustin Hoffman), who tries to discover what kind of novel Harold is in. Freakin' hilarious!