When the military enlists a ragtag group of monsters to save the world from destruction by aliens, it's Reese Witherspoon to the rescue! Reese and an all-star cast including Kiefer Sutherland and Seth Rogen lend their voices to mankind's assorted friends and foes in 'Monsters vs. Aliens' -- and ET has your first-look behind the scenes of DreamWorks' new animated flick!

"This is a really cool group of monsters and it makes me laugh so hard," Reese says of her colorful comrades. In the film, Reese voices Susan Murphy, an ordinary human transformed by meteor-gunk into the towering 49-feet-11-inches-tall "Ginormica."

Joining her as earth's last line of defense against a destructive alien-robot are a one-eyed blue blob, B.O.B. (Seth); the brilliant Dr. Cockroach, Ph.D (Hugh Laurie); and the half-ape half-fish Missing Link (Will Arnett). Other A-list voices featured are Kiefer as General W.R. Monger, Stephen Colbert as the President and "The Office"'s Rainn Wilson as the villainous Galaxhar.

Be sure to catch 'Monsters vs. Aliens' in true 3-D when it zooms into theaters on March 27, 2009!