Reese Witherspoon's holiday comedy "Four Christmases" focuses on the hoops her and Vince Vaughn's characters have to jump through to satisfy the couple's blended families. It turns out its something with which Reese, who has two young children with ex-husband Ryan Phillippe, has had some firsthand experience.

"I think the most important thing is to be a grownup," Reese tells People.com. "And not let any kind of feelings affect how you deal with your children."

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Witherspoon, 32, and Phillippe, 34, make sure the lines of communication are open, and that the kids always come first. "I think the most important thing is the children," says Witherspoon. "What do they want, what makes them feel comfortable?"

VIDEO: Reese and Jake talk about their last film, Rendition.

Witherspoon has been dating Jake Gyllenhaal, 27, for several months. But when asked if she saw marriage in her future, the star demures. "I don't know," she tells People.com. "I don't think about it much."