The actors star as the same assassin at different points in his life in the time-bending new film Looper, which is directed by Rian Johnson.
Willis has suggested that Gordon-Levitt was perfectly cast in the film particularly because of his physical resemblance to the Die Hard actor.
"I was sitting across from Joe across a table. I was supposed to act and get all my lines right, but I just found myself looking at him and thinking how weird it was," he joked, according to Flicks and Bits.
Willis went on to say of Gordon-Levitt: "It's really a strange thing to see someone that looks like a young version of yourself. He's a great actor, I love his work and I just love what he did in this film, Looper.
"He picked up some of my cadence of speaking, which was odd, and yet, really cool at the same time."
Willis also touched on what initially drew him to Looper.
"I never judged my character of old Joe, I was just so caught up in those goals of what old Joe wanted to do in the film that I took it for granted that I had to hurt other people – and really innocent people," he said of the film.
"I never thought that was a bad thing while I was doing it. When I saw Looper, I was emotionally moved some of things I was doing for my characters goals, sort of loving goals."
Looper opens on September 28 in the US and in the UK.
Watch a trailer for Looper below: