The Dark Knight Rises actor, who plays a younger version of Bruce Wills in the time travel movie, sat through two and a half hours of make-up every morning to achieve the Die Hard star's look.
"They did such amazing work," Gordon-Levitt told Zap2it. "I would look in the mirror and think that was my face!
"It got to the point that on weekends, when I had my normal face, it was weird to me," he continued. "To have a different face, it changes everything. That's what I love about acting."
The actor went on to talk about how he prepared for the role.
"Most of preparing for this movie was watching Bruce Willis movies, listening to his voice, taking the audio out of his movie to listen to them on my iPod," he explained.
"And then just hanging out with him," Gordon-Levitt said. "And then there were two and a half hours of special effects make-up every morning."
Looper opens in the US and UK on September 28.
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