Sissy Spacek in her Oscar-winning performance as Country singer, Loretta Lynn, in 1980's "Coal Miner's Daughter".

This scene is a personal favorite of mine not only for my love of a good cinematic breakdown, but because it shows how much Sissy Spacek understands the character she is playing. She completely breaks it down for the audience by using subtle movements and the patterns in which she takes her breaths.
It's not the performer Loretta Lynn speaking in this scene, but the woman that happens to be Loretta Lynn. You don't really see acting like this in movies nowadays- where an actor puts their whole body into it without going completely overboard. There's no vanity in this performance.

From the unwavering stomp she makes up to her stage, to her rattled, vulnerable eyes before she calls out for her husband, Spacek shows the pressures this woman feels as a mother, wife and performer.

I don't always think awards are relevant in a actor's performance, but I do feel that it was more than justified that Spacek received the highest honor in film for this performance.