Hopkins joined the Die Hard star and castmates John Malkovich and Dame Helen Mirren in the sequel to 2010 action movie Red, and Willis admits he was mesmerised watching the Oscar winner at work.
He says, "The day that we found out that Tony was going to do the film, I was like, 'Yeah! This is so good!' Everybody was very excited - Dean (Parisot) too, Dean is our director and is a very low-key guy but he was very, very excited that we got Tony. The other assorted kooks and characters who are in this film were all so happy to watch Tony work. There were times when I kind of stopped acting - I was just a fan for a few moments on the set."
Mirren adds, "When we did the first scene with him, we did a little bit of rehearsal but not much, and then bang, we're into shooting. We're on Tony first and Tony does this incredible performance just like that. Bruce and I were just both standing there with our jaws open, going, 'Oh my God, that's the competition. Follow that!' He was amazing."
Meanwhile, Hopkins insists he didn't have to think twice about joining the cast: "My agent phoned me and said, 'Would you like to do Red 2?' Bruce Willis? John Malkovich? Yeah! I'd met Bruce before just briefly, and he gave me a great welcome, everyone did."