Forgetting Sarah Marshall is a comedy never forgetting that “screwball” is a compound word. It reminds you with endless variations on the first half and several glimpses of the second, courtesy of writer/actor/exhibitionist Jason Segel. He’s one to watch in the future, when you aren’t covering embarrassed eyes.

Segel has more going for him than pot jokes and leering at women. His first screenplay displays uncommon respect for the DNA of romantic comedy, making the genre’s conventions modernly crude with old-fashioned sweetness.

Who can’t like Peter Bretter (Segel), for whom life is an oversized bowl of Froot Loops? He’s dating TV star Sarah Marshall (Kristen Bell) while composing her crime-lab show’s musical score (more accurately, sinister one-note tones). Sarah returns from a trip with news that’s she’s dumping Peter, who is too shocked to put on clothes, the first of numerous gasp-laughs to come.

Depression envelops Peter, ruining his work – Seinfeldian noodling isn’t appropriate for autopsies – and straining relations with his stepbrother (Bill Hader). He must escape, and where else but where Sarah always wanted to go: a lush Hawaiian resort. Thing is, she’s also there with her new lover, a British rock star named Aldous Snow (hilarious Russell Brand), possibly Spinal Tap’s groupie love child.

It is a classic triangle: hopeless romantic, object of affection and undeserving rival. Segel completes the screwball package with wacky sidekicks (30 Rock’s Jack McBrayer, Apatow regulars Hader, Jonah Hill and Paul Rudd) so the leads don’t need to carry the funny, and a procession of crises that can’t logically be overcome. Yet he makes us hope they will, which is what a screwball wannabe like Leatherheads doesn’t do.

Forgetting Sarah Marshall opens Friday. Read the full review Thursday in Weekend.