Al Franken, comedian, writer, talk radio host and U.S. senator, made an interesting turn as co-writer of the romantic drama When A Man Loves A Woman, starring Meg Ryan as an alcoholic struggling to keep her marriage together.
The film is supposedly based on Franken's real life relationship with his wife, who is a recovering alcoholic.
Leslie Dixon, usually a comedic writer for films such as the Hairspray remake, also wrote the sci-fi thriller Limitless, about a man who becomes addicted to a powerful mind drug.
Aaron McGruder, creator of the comic strip turned animated series The Boondocks, also co-wrote the non-comedic war film Red Tails, about the Tuskegee Airmen in World War II.
If this can be extended to television, Stephen J. Cannell was not limited to cop dramas and adventure shows - as one newspaper article pointed out, do The Greatest American Hero and Baa Baa Black Sheep have any real similarities other than both involving flying?
Vince Gilligan's background was in comedy before he joined The X-Files' writing staff, dividing his time between writing comedic and serious episodes. As his own show Breaking Bad gets increasingly dark, it deviates further from his type, if it hasn't redefined it altogether.
Gilligan was also one of the (many) writers of Hancock, which combines comedy and science fiction.