Marilyn Monroe and Cary Grant in Monkey Business (1952).
Carole Lombard in Howard Hawks's Twentieth Century (1934). You can see Carole's scar quite clearly in this shot and I love it; it's such a beautiful reminder that even a woman this glamorous wasn't "perfect," whatever that means. She was better than perfect; she was real and brave and funny and magnificent.
Jean Arthur in Too Many Husbands (1940).
Irene Dunne in The Awful Truth (1937).
Preston Sturges's cameo in Sullivan's Travels. He's the man with the rather unruly dark hair standing behind Veronica Lake.