EXCLUSIVE: Ben Affleck Says Brother Casey's Golden Globes Win Was the 'Most Memorable Experience in My Life'

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Casey Affleck took home the Golden Globe for Best Performance by an Actor in a Motion Picture, Drama, on Sunday, for his performance in Manchester by the Sea, and the only person who may have been more thrilled by the win than Casey was his brother, Ben Affleck.

The Oscar winner stopped to talk with ET at the premiere of his new film, Live by Night, at the TCL Chinese Theatre in Hollywood on Monday, where he opened up about the joy and pride he felt in watching his younger brother win the coveted award.

WATCH: Ben Affleck Hopes He and Casey Will Be the First Brothers to Win Golden Globes and Oscars

"The overwhelming sense I had last night, watching my brother win, and believing in him for so long and knowing how talented he is… it was the most memorable experience in my life," the 44-year-old star shared. "I'm thrilled. I wish him the best, but already what he's done is amazing."

Ben was at the show where he was able to cheer his brother on in person, and the pair ended up celebrating the big win well into the night -- but, as the father of three admitted, he wasn't able to stay out too late.

"I had to get up and take the kids to school, so responsibilities were still calling," said Ben, who shares three children -- Violet, 11, Seraphina, 8, and Samuel, 4 -- with his estranged wife, Jennifer Garner.

WATCH: Casey Affleck Gushes Over Michelle Williams During His Golden Globe Win

Ben's best friend and frequent collaborator, Matt Damon, who also served as a producer on Manchester by the Sea, celebrated the win with the brothers. The Martian star also came out to the Live By Night premiere to support his friend, who had nothing but love and praise for his long-time compatriot.

"I can't tell you how valuable it is to have somebody who's been through things with you, ups and downs, who knows what your life experience is like, who can identify with that," Ben explained. "It's an incredibly valuable friendship and it's very precious and so is my friendship with my brother. I don't know what I would do without those guys."

Live by Night
hits theaters this Friday.

WATCH: Casey Affleck Talks First Golden Globe Win, Jokes Brother Ben Displays His Awards ‘In His Window’

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