There comes a time in the career of every "serious comedian" when he or she must do a "serious comedy". After the deliriously funny "Talladega Nights: The Ballad of Ricky Bobby", Will Ferrell gets his "Truman Show" with "Stranger Than Fiction", an uneven dramedy about a man who begins to hear voices in his head - not voices like "Son of Sam", mind you, but rather the dulcet tones of Emma Thompson narrating his lifestory. Though this is an inherently funny premise, it doesn't lend itself to a full length movie; it's more of a one-joke "Saturday Night Live" sketch. In fact, there was a "Carol Burnett Show" routine which featured an author struggling at a typewriter as his exasperated characters acted out his whims in a thought-bubble over his shoulder. As with many things, Ms. Burnett had it right.

That's not to say that "Stranger Than Fiction" doesn't have it's moments. In fact, there is quite a lot to like here. When at last we meet Ms. Thompson's character, she's revealed to be a blocked author writing a story about Harold. In an inspired twist, the author is struggling with her decision to kill Harold off. Queen Latifah is assigned to her by her publisher as a kind of guardian angel who accompanies her on a morbid search for the perfect death for Harold. And the cast just gets better and better from there. Linda Hunt shows up briefly as a psychiatrist who recommends Harold to a literary professional (Dustin Hoffmann, in a happily subdued performance). Maggie Gyllenhaal, as always, is a breath of fresh air as Harold's unlikely (very unlikely) love interest.

Director Marc Forster ("Stay", "Monster's Ball") has a lot of fun with the script by Zach Helm, particularly in one flirtatious scene aboard a double-length bus and in the way the film visualizes Harold's OCD as a series of internet-esque rollovers. But before you know it, the film becomes a sweet, if ultimately dull, love story and the voice-over plot disappears. The charm of Mr. Ferrell and Ms. Gyllenhaal carries the love story until Mr. Forster reverts to schmaltz (see "Finding Neverland") and the film devolves to a needlessly morose climax.

When comedians - particularly wacky ones like Mr. Ferrell - opt for "serious" roles, they have a lot to contend with. Typically, the first few times, they tend to over-compensate in other direction to point of stoicism (remember, it wasn't until "Eternal Sunshine of the Spotless Mind" that Jim Carrey got it right). Will Ferrell does low-key well, but he hits one note and stays there; the performance lacks variation. And by virtue of the premise (we hear everything Harold is thinking), Mr. Ferrell's character needs no inner life.

Though there are enough interesting moments to keep it refreshing, Mr. Helm's script never rises above its conceit to reach the giddy Charlie Kaufman heights to which it aspires. Despite an odd tangent into guitar playing, the pop metaphysical morale boils down very simply to: "hey, you could go anytime, so live your life." True enough, but if that was the film's point, then - as is the case with most films - we didn't need the voice-over.