The actor explained that he thoroughly researched Willis while preparing for Looper in order to accurately replicate the Die Hard star's distinct speaking style.
"I studied Bruce Willis's movies, and I would take the audio from some of his movies and put them on my iPod so that I could listen to him," Gordon-Levitt said, according to Flicks and Bits.
He further explained: "Bruce actually recorded himself doing some of my monologues and sent me the tapes so that I could listen to that, and that was all really useful.
"But I think the most important thing for me was just getting to know him, hanging out having dinner, talking about whatever, that was where I think I really learned what I wanted to do with the character."
Willis previously claimed that he was very impressed with Gordon-Levitt's performance in the sci-fi film.
"He's a great actor, I love his work and I just love what he did in this film, Looper," Willis commented. "He picked up some of my cadence of speaking, which was odd, and yet, really cool at the same time."
Looper opens on Friday in the US and the UK. Watch a trailer below: