Marguerite Roberts adapted both True Grit and Norwood for the screen.

The film of True Grit was true to the novel with only a few changes, the most important being that Texas Ranger LaBoeuf dies in the shootout with the outlaws and Rooster Cogburn comes to visit Mattie after she recovers and returns to her home near Dardanelle. The 1969 film starred John Wayne, Kim Darby, Robert Duvall, and Glenn Campbell.

Norwood (1970) is an entirely different matter. Although an ex-Marine is the protagonist and many of the characters in the screenplay bear the names Portis gave them, the story line gathers some Hollywoodish complications, many of them worked in to enable Glenn Campbell to use his musical talents. The most radical departure involves a performance by Norwood in a Greenwich Village coffeehouse.

Neither Marguerite Roberts nor Portis had a hand in the screenplay for Rooster Cogburn (1975), a vehicle for...