Charles Brackett and Billy Wilder
Although The Lost Weekend is based on Charles R. Jackson's 1944 novel of the same name, the film's Academy Award-winning script differs from its source material in a few ways. And these aren't just the differences that happen with any movie adaptation—they reflect some deeply personal experiences of The Lost Weekend's director and co-screenwriter, Billy Wilder.
When he decided to direct a film about alcoholism, Wilder had just finished working on 1944's Double Indemnity, a highly regarded film he directed and co-wrote with Raymond Chandler. Chandler, in case you don't know, is one of the most legendary detective fiction writers of all time—he wrote The Big Sleep, among countless others.
He also happened to be a raging alcoholic, which made scriptwriting a painful process for Wilder.
After stumbling across a copy of the novel, Wilder was immediately inspired to adapt it to film, seeking to (according to his biographer, Maurice Zolotow) "explain Raymond Chandler to himself." That's no small task. Still, Wilder set to work, employing the help of producer and longtime collaborator Charles Brackett to write the script.
Although the film stays true to the book in a bunch of ways, there are some notable aspects that are changed, mostly due to the notoriously prudish Hays Code restrictions of the time. For example, the novel features a subtle-but-clear subtext that Don's gay, but that's not present in the film at all.
The most notable difference from the book, however, is the ending. The novel closes on an unambiguously sour note for Don: he picks up drinking again immediately after the weekend ends. In contrast, the movie has a happy ending—with Don turning to writing instead of booze—though it leaves the possibility open that he'll turn back to his old ways at some point.