Written and directed by Rian Johnson, Looper is a time-travel movie in which a killer (Joseph Gordon-Levitt) discovers that one of his targets is his future self (Willis).
"It's better than anything I've ever done," Willis told Esquire.
"Rian did an amazing thing," he continued. "He conceived an original story. He wrote it, sold it, stuck with it, directed it, and finished it. That's just tough to do in this town. Someone always weasels into the process. That didn't happen here."
Gordon-Levitt has also said that Looper represents some of his best work as an actor, insisting that he really transformed himself for the role.
The 31-year-old underwent extensive time in the make-up chair every morning in order to achieve Willis's look. He said that even his own mother had trouble recognising him by the time the transformation was complete.
Watch the trailer for Looper below: