Presenting the 17th Academy Awards' nominees for Best Supporting Actor:
Hume Cronyn, The Seventh Cross
Barry Fitzgerald, Going My Way
Claude Rains, Mr. Skeffington
Clifton Webb, Laura
Monty Woolley, Since You Went Away
Cronyn is a man in Nazi Germany who must choose between protecting himself and helping a friend, a political refugee. Fitzgerald is an old and old-fashioned priest with a crumbling parish. Rains is a successful Jewish businessman who marries a woman who loves his money more than him, but he's hopeful that'll change. Webb is a bitchy columnist whose protege's murder is investigated. Woolley is a stuffy retired colonel boarding with a middle-class family, nursing a strained relationship with his grandson.
This was always going to be Fitzgerald's Oscar:
The fact that he was nominated for the exact same performance in Best Actor tells you so. At the time, Academy bylaws permitted an actor to be considered for whatever category, and he happened to score enough votes to place in both categories. Afterward, rules were changed so that an actor could only be nominated in the category submitted by their studio, but that got confusing, too: a clerical error in 1963 led to the entire cast of Cleopatra being submitted as leads, while Peter Sellers' multiple roles in 1964's Dr. Strangelove led to four simultaneous campaigns: one in Lead for all three roles together, and three in Supporting for each role individually. So then the rules were changed again: you could vote for a role in whichever category, but the category they got the most votes in first would be their nomination: thus, Kate Winslet in The Reader went from being campaigned all 2008 as Best Supporting Actress, only to wind up in Best Lead Actress.
So that's how a Supporting Actor won the Oscar and changed the rules.
It's not, admittedly, the best lineup. My rankings, from fifth to first:
