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Turning 40 can't be that bad, can it? That's the question director Judd Apatow explores in This is 40, his new sorta-sequel to Knocked Up, and we have a first look behind the scenes at the new relationship comedy starring Paul Rudd and Leslie Mann.

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"It's tough when you're married to somebody to just tolerate them sometimes," says Paul. "You love them but you also kind of can't stand them many times, and so this is a couple that's going to try and fix a lot of the things -- it's a big moment, we're both turning 40, and let's make what's good better, and let's make what's bad good."

In theaters December 21, This is 40 expands upon the story of Pete and Debbie from Knocked Up, picking up about five years later to see how they are dealing with their current state of life with kids, friend and parents.

"I guess they're in the middle of a mid-life crisis," says Leslie. "It's a stage that they have to get through, but getting through it sucks, but they know that there's light at the end of the tunnel if they just stick with it, but sticking with it is like the hardest thing ion the world to do."

Video: A New Look at 'This is 40'

The new comedy also stars John Lithgow, Chris O'Dowd, Megan Fox, Iris Apatow, Maude Apatow, Melissa McCarthy, Robert Smigel, Charlene Yi and Albert Brooks.