Thelma Ritter has an even larger role in a better movie in 1951, The Model and the Marriage Broker. She plays the marriage broker, not the model, in case you were wondering, and has a wider range of emotion to portray than in any other film of hers I've seen. Thelma makes my list of top five nominees for best actress.
Lawrence, I'll agree that 1951 is not a great year for numbers in the supporting actress category. Most of my candidates are from less familiar films like Decision Before Dawn, Laughter in Paradise, The Tall Target, Westward the Women, Teresa, The Blue Veil, and Goodbye, My Fancy.
1952 will present us with some more problems. I think Colette Marchand plays a lead in Moulin Rouge, even if she does disappear halfway through the movie. Her effect on Jose Ferrer is really what the film is about.
Are Paul Douglas and Robert Ryan leads in Clash by Night, or is Stanwyck the only lead?
Richard Burton is, of course, a lead in My Cousin Rachel.
I think Alberto Sordi has a supporting role in The White Sheik, even if he does play the title character.