It's a Great Feeling (1949)

A waitress in the Warner Bros. cafeteria, Judy Adams (Doris Day) is picked by actors Jack Carson (Jack Carson) and Dennis Morgan (Dennis Morgan) to star in their next picture. Several problems arise from the decision, though, specifically keeping Adams' true identity a secret by dressing her in various costumes to disguise her from the studio bosses - and that Carson and Morgan are both falling for her. It's enough to make a small-town girl ditch her Hollywood dreams and go racing back home.