During his acceptance speech, Casey gushed over his co-star Michelle Williams' performance in the film.
"There's no award that belongs to just one actor," he told the audience. "I wish Michelle could be on stage with me because she made most of it work. She's perfect."
Among the many other names that Casey mentioned were the film's producers, Matt Damon and Kevin Walsh.
"To Kevin Walsh and to Matt Damon for giving me the opportunity, your support, your friendship," he said before taking a playful shot at Damon. "I suspect you won't be passing on any more movies...but I'll take this one."
Before getting played off by the house band, Casey sent a sweet message to his family.
"I just want to say that despite how I might think I’m in charge at my house, it’s my kids who give me permission to do this, because they have got the strength of character to keep at bay all the noise that sometimes surrounds people who live publicly, and to let me travel for months at a time," he shared. "And so I love you Indiana and Atticus."
In another special stage moment, the Boston native praised his wife, Summer Phoenix, for giving him "just about every good acting idea" that he’s ever had.
"Thank you very much," he said. "I love you."
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Like any good sibling, Ben was cheering Casey on from the audience.
Although Casey revealed to ET that Ben -- who previously won two Golden Globes for Argo and one for Good Will Hunting -- wouldn’t dish out any advice, the 44-year-old actor was hoping that his younger brother would win.
And he wants the sibling winning streak to continue with the Oscars.
"I was trying to see if there were any brothers that have won Oscars for separate movies for separate categories in separate years, and I'm not sure there are," Ben told ET's Carly Steel on Friday. "Warren Beatty and Shirley MacLaine both won, and they're siblings, but they're not brothers."
Find out what else Ben had to say about Casey below.