Stage Fright (1950)

The police think actor Jonathan Cooper (Richard Todd) is a murderer, and now they're on his tail. He asserts that it was his lover, the famous actress Charlotte Inwood (Marlene Dietrich), who killed the victim (not coincidentally, her husband). He seeks shelter with his ex-girlfriend Eve (Jane Wyman), a budding actress. Convinced Jonathan is innocent, Eve plays detective and assumes multiple disguises. But once she is entangled in a web of deception, she fears everyone in fact wears a mask.