Maxwell Anderson, Del Andrews, George Abbott, C. Gardner Sullivan
How many writers does it take to screw in a light blub? Four: one to screw in the light bulb and three to provide editorial notes on his character development and the thematic implications of lefty loosey, righty tighty.
How many writers does it take to adapt one bestselling novel for the silver screen? Same answer…minus the lightbulb.
Maxwell Anderson was hired first to write the screenplay because of his work on the 1926 WWI comedy-drama What Price Glory. According to Garry Wills, "[h]is first attempt, guided by the film's producer, failed to ascend above easy movie clichés, but he, screenwriter Del Andrews, and Milestone were able to refashion it, with the help of theatrical genius George Abbott" (source).
After All Quiet, Anderson continued to write plays—many of which would be adapted for cinema—as well as other screenplays. He would work with Milestone again, penning the 1932 screen adaptation for Rain.
Before assisting Anderson on All Quiet, Del Andrews had written a few screenplays, but his main job in Hollywood was as a director. During the 1920s, he received directing credits for a whooping forty-two films, several of them shorts and westerns (source).
George Abbott also received a screenplay credit for All Quiet. He worked as a producer, director, playwright, screenwriter, and film director—resulting in an impressive (if somewhat crammed) business card. During his storied career, Abbott would win five Tony Awards and a Pulitzer Prize for Drama for Fiorello! (source).
Oh, and let's not forget C. Gardner Sullivan, who received a script supervisor credit on the film. Sullivan began as a newspaper reporter before making the jump to scripts. He "began producing films for [Cecil B.] DeMille in the mid-'20s" and "also produced films for his own company" (source).
And, of course, we should probably give Erich Maria Remarque his dues for writing the novel the screenplay was based on. But you can read about his accomplishments in our All Quiet on the Western Front discussion here.