Bo Burnham's comedy-drama took the prize for Original Screenplay, beating out Green Book, Vice and Roma -- whose screenplays are all nominated for Oscars -- as well as A Quiet Place.

Burnham was also an upstart winner earlier this month at the Directors Guild of America awards where Eighth Grade won for best first-time feature.

In another upset, the top prize for Adapted Screenplay went to Can you Ever Forgive Me? written by Nicole Holofcener and Jeff Whitty about a biographer who turns to a life of crime.

Melissa McCarthy plays Lee Israel, the writer whose memoir is the basis for the film of the same name.

Read also: 'Black Panther,' 'Star is Born' among AFI's top films of 2018

Holofcener and Whitty, who are nominated for an Oscar, beat out BlacKkKlansman and A Star is Born -- also contending for Hollywood's biggest prize in the adapted screenplay category next week -- as well as Black Panther and If Beale Street Could Talk.

Best documentary screenplay went to Bathtubs Over Broadway. Ozzy Inguanzo and Dava Whisenant won for their chronicle of Late Show with David Letterman writer Steve Young's fascination with corporate musicals produced only for company employees.

In the television category, FX's The Americans won for best drama, while Prime Video's The Marvelous Mrs Maisel took the comedy prize and HBO's Barry topped the New Series category.

The WGAs are one of Hollywood's last major prize giving ceremonies ahead of the Academy Awards. 

They are not considered as reliable a predictor of Oscars glory as the other guilds' prizegiving nights, as members can only shortlist scripts written under WGA guidelines or those of several international partners.