From an interview with Robert De Niro in the January 1989 issue of Playboy:
PLAYBOY: Do you remember your first experience before the cameras?
ROBERT DE NIRO: There was some little thing I did that I don’t know whatever happened to. Some walk-on for an independent film: I walked in and ordered a drink at a bar.
I remember a bunch of other young actors hanging around, moaning and bitching, all made up, with pieces of tissue in their collars; it was the kind of thing you always hear about actors—where they’re just silly or vain, complaining back and forth, walking around primping, not wanting to get the make-up on their shirts.
PLAYBOY: So you didn’t exactly feel as if you had found a home.
ROBERT DE NIRO: No, I didn’t want to be around those people at all. I just walked in and walked out. I was nervous, though, just to say the line “Gimme a drink.” It makes me think of that joke: “Hark! I hear the cannon roar!” You know that joke?
ROBERT DE NIRO: I’m surprised you never heard it; it’s a famous actor’s joke.
This guy hasn’t acted in about 15 years, because he always forgets his lines, so finally he has to give it up. He’s working in a gas station and gets a phone call from someone saying that they want him for a Shakespearean play—all he has to do is say, “Hark! I hear the cannon roar!” He says, “Well, God, I don’t know.” The director says, “Look, it’ll be OK. You’ll get paid and everything.” So he says, “OK, I’ll do it.” The play has five acts and he has to go on in the third act and say, “Hark! I hear the cannon roar!” That’s all he has to do. So he rehearses it when he’s in his apartment: “Hark! I hear the cannon roar! Hark! I hear the cannon roar! Hark! I hear the cannon roar!” Every variation, every possible emphasis. They’re into rehearsal, and he’s got it written on his mirror: “Hark! I hear the cannon roar! Hark! I hear the cannon roar! Hark! I hear the cannon roar!” And so on. Finally, comes opening night, first act, no problem. Second act, things go fine. Audience applauds. Stage manager says, “You have five minutes for the third act.” He tells him to get backstage. His time comes, he runs out, muttering to himself, “Hark! I hear the cannon roar! Hark! I hear the cannon roar! Hark! I hear the cannon roar!” And as he runs out, he hears a big brrrooooom!! Turns around and says, “What the fuck was that?”
Source: I found the interview posted in parts online; the above was included here. According to another source, the interview was conducted by Lawrence Grobel and the material is Copyright 2002 Playboy.