Sheen said during an interview at the Dubai International Film Festival that the scene was filmed on his 36th birthday, and that he'd been drinking all day in celebration. He added, "I'm an alcoholic, I got sober in 1989...and I was drinking very heavily at that time."
When it came time to shoot the scene, Sheen said, "I was so dangerously drunk I could hardly stand up." Still, he committed to the scene's objective, which was to establish his character, Captain Benjamin L. Willard, as dangerous and reckless. "I went very deep into my own personal pain and alcoholism and self-loathing and all of the things that make up a human being," he said.
The most famous part of the scene, of course, is when he punches the mirror, bloodying his hand. That, Sheen revealed, was a drunken mistake, as he was closer to the mirror than he realized. He said director Francis Ford Coppola wanted to stop the filming immediately, but he refused, wanting to continue wrestling the demon on camera. "And so I did and I just showed a completely broken, desperate guy, which was a reflection of who I was at the time."