After the war, Derek had a small role in A Double Life (1947) when he was approached by Humphrey Bogart, who renamed him John Derek and cast him as Nick (Pretty Boy) Romano, an unregenerate killer, in Knock on Any Door (1949), a socially conscious melodrama directed by Nicholas Ray. Derek was recognized as a talented newcomer, "plainly an idol for the girls", as Bosley Crowther put it in a review for The New York Times. The Los Angeles Times called him "a handsome hot-eyed newcomer who makes the case for this product of the city's slums - 'live fast, die young and have a good looking corpse' - all too fascinating for everybody's comfort."