'Pacific Rim' Was a Monster Undertaking

Monsters are very real and working very hard to make mankind extinct in Guillermo del Toro's hugely anticipated summer popcorn flick Pacific Rim, in theaters Friday, and the stars of the film --- including Charlie Hunnam, Charlie Day, Idris Elba, Ron Perlman and Rinko Kikuchi -- explain to ET that everyone was put to the test in making the epic movie -- and in some cases flirted with the possibility of serious bodily injury!

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"It was an amusement park from hell. We built the machine in a way that put a lot of stress on the [actors'] backs, their muscles, their knees -- we needed to create a system that prevented their knees from snapping or their waist from cracking," says Guillermo with a straight face about the complicated "robot head" cockpit rig that the stars were required to perform in. "The [monster] would throw a punch, and you would see them react to the punch for real."

In Pacific Rim, a dimensional rift from beneath the sea unleashes a torrent of alien monsters, known as Kaiju, intent on wreaking a path of destruction on Earth. With the creatures impervious to conventional weapons, the only way to defeat them are with Jaegers – giant mechanical robots piloted by two people whose minds have merged with the machine and each other. But with the Jaeger program scheduled to be shut down and a greater number of monsters moving in for the attack, can a failed pilot (Hunnam) and an untested candidate (Kikuchi) save the human race?

"It's not easy at all – our suits were very, very heavy," says Rinko of the demanding flight suits the Jaeger pilots would wear in the film. "But it looks cool and when [my character] wears the suit. It looks totally bad-ass, you know?"

Related: 'Pacific Rim' Tome Teases Look at New Monsters

Watch the video to see more with the Pacific Rim stars – and who they would choose to be their real-life Jaeger co-pilot if we were really attacked by alien monsters!