The Canadian author Iain Reid writes electrifying, if unsettling, novels about what happens inside relationships. His first novel, “I’m Thinking of Ending Things,” was adapted for the screen by Charlie Kaufman. Now, his second novel, “Foe,” is coming out as a film, starring Saoirse Ronan and Paul Mescal. Iain wrote the screenplay himself. He talks to Tom about turning his dystopian novel into a movie filled with emotion, what it’s like to write novels that get adapted for the screen, and what draws him to writing about relationships.