EXCLUSIVE: Kate Beckinsale Reveals the Secret to Co-Parenting With Ex Michael Sheen

Kate Beckinsale dishes on spending the holidays with both her ex, Michael Sheen, and his girlfriend, Sarah Silverman.

Kate Beckinsale and her ex, Michael Sheen, may have one of the most amazing relationships post-split that we've ever seen.

The two dated for eight years before breaking up in 2003, and are parents to their 17-year-old daughter, Lily Sheen. ET recently spoke to the 43-year-old British actress while she was promoting her new film, Underworld: Blood Wars, when she shared that the two still spend holidays together, as well as with Sheen's girlfriend, comedian Sarah Silverman.

"Well, we had Thanksgiving at his girlfriend's house and they're having Christmas at our house," Beckinsale tells ET's Ashley Crossan. "We like hanging out with each other, so it's not like, 'Oh, who gets Lily?' or whatever. It's like, 'Whose house are we going to be at?'"

WATCH: Michael Sheen and Kate Beckinsale Adorably Embrace Upon Hearing Their Daughter Lily Got Into College

Beckinsale reveals the secret to their idyllic relationship.

"Well one, pick well," she says bluntly. "And two, I think we both really felt like, we really agreed on what would be the right thing for Lily. And I think, you know, not hating each other and fighting and all that would be awful for her. I think we've also got a really similar sense of humor, and I do find that humor can save an awful lot of things and help a lot of things."

"I just really like him," she adds. "He's family. I've known him since I was 21, so he's proper family now."

Just last week, Beckinsale Instagrammed an adorable picture of her and 47-year-old Sheen emotionally embracing after finding out Lily got into college.

"[Lily] did say, 'Why didn't you put that I got into the college that I wanted to get into? It sounds like you were so relieved that I got into any college at all, like you have no confidence in me whatsoever,'" she hilariously revealed. "But yeah, I'm really excited about it. I'm really excited for her."

And it appears Beckinsale won't have to worry about empty nest syndrome. The actress says she's looking forward to now having more downtime for other projects, and she's already started writing her own screenplay.

"It's nice as well because it gives me a little bit more room to write and to do other things that I've not necessarily been able to do," she explains about entering a new phase in her life. "That's obviously changed in the last couple of years because [Lily's] not dying to hang out with me every minute of the day anyway, but I'm pretty excited about it."

NEWS: Michael Sheen Says Ex Kate Beckinsale and Girlfriend Sarah Silverman Gang Up on Him -- They 'All Just Make Fun of Me'

These days, Beckinsale's been busy promoting Underworld: Blood Wars -- the highly anticipated fifth installment in the Underworld franchise -- which hits theaters on Jan. 6. Watch the trailer below: