While at an academy for show business, a teacher told her that the casting director Barbara L. Claman was looking for streetwise kids to appear in a new Hollywood film. Manz turned up unannounced at Claman’s office, “smoking and looking all of 10 years old” but according to Claman “she had that special quality we wanted.” This introduction eventually lead in 1976 to Manz being selected at the age of 15 by Terrence Malick to act in his second film, "Days of Heaven," as a streetwise orphan who joins her older brother and his lover when they flee Chicago in 1916, and find work, then refuge, with a wealthy Texas farmer. The film was released in 1978 due to its lengthy editing. Manz's part was initially smaller, but Malick was so impressed by her that he made a last-minute decision to have her provide an unscripted, improvised narration. Manz told interviewers, years later, that, “I just watched the movie and rambled on,” and that, “They took whatever dialogue they liked.” She received excellent reviews, with critic Roger Ebert saying, “Her voice sounds utterly authentic; it seems beyond performance.”