Peter Sarsgaard and Maggie Gyllenhaal are, in fact, the real thing.

And after more than 14 years of marriage, the Memory actor chalks up his long-lasting relationship with the actress to one basic rule.

"Tell the truth," Peter told E! News' Francesca Amiker in an exclusive interview. "Isn't that all there is? It's pretty simple. It's not that complicated. Isn't that the secret to any friendship or relationship?"

For Peter—who shares kids Ramona, 17, and Gloria, 11, with Maggie—honesty is, indeed, always the best policy.

"Otherwise, even if it's a nice lie, even if it's a lie to prop someone up or help them avoid what they're trying to avoid, you're not living in reality," he continued. "And that's the only place to live in."

And these days, they're living in—and relishing—the holidays and their annual traditions.

"There's a church that we go to that has a midnight mass that I don't go to any other time throughout the year," the 52-year-old, a close friend of Maggie's brother Jake Gyllenhaal, explained. "The midnight mass there is extraordinary, and Maggie and I are sometimes able to sing."

As for the ultimate present he thinks you can give to someone? Much like his character opposite Jessica Chastain in Memory, it involves allowing another person to simply be themselves.

"Being a mirror, just reflecting what you see accurately," he noted. "That's the greatest gift you can give anyone. A partner, a friend, anything."

Memory hits theaters Dec. 22 in New York and Los Angeles, followed by a nationwide release Jan. 5.

Until then, to see how more of your favorite stars are getting into the holiday spirit, keep reading…

The model gave a look at husband Joshua Kushner lighting the menorah for Hanukkah.

The Love Is Blind star shared a photo of her family's bright Hanukkah celebrations.

The Lord (Disick) hath spoken.

The Keeping Up With the Kardashians alum captioned this Dec. 7 snap of his home, "HAPPY HANUKKAH MY JEWS!!!"

Khloe shared a holly jolly look at kids True and Tatum Thompson, along with their cousin Dream Kardashian.

The Glass Onion star rings in the Festival of Lights with her 5-year-old daughter Rani.

The Live With Kelly and Mark cohost set up her Christmas tree on Dec. 1, joking on Instagram, "She’s dressed to impress."

The singer got the party started on the first night of Hanukkah by letting her daughter Willow, 12, light the menorah. 

The Jersey Shore star and her husband Jionni LaValle took in some GTL—gym, tan, lights—during this holiday season.

The Jeopardy! host ushered in the first night of Hanukkah with not one, but three menorahs. 

The Today anchor got some help from daughters Haley, 6, and Hope, 4, to set up the Christmas tree.

The country star shared a sparkling look at the first Christmas tree in her new home, adding, "i'm gonna cry."

Reese Witherspoon and Ryan Phillippe's daughter asked if it's "still too early for a holiday sweater" on Nov. 13. Clearly not!

The late night talk show host and the Real Housewives of Beverly Hills star lit the menorah in honor of Hanukkah during the Dec. 6 episode of Watch What Happens Live.

The Kardashians star's kids True and Tatum got into the festivite spirit with matching Christmas pajamas from Zip N’ Bear.

The country couple's four daughters Lillie Carolina, 2, Lennon Love, 3, Ada James, 6, and Willa Gray, 8, helped decorate the tree.

"It's the most wonderful time of the year," Lauren wrote on Instagram Nov. 24. "Only 3 broken ornaments and maybe a few tears shed-the sweet season of life where we all sleep really hard at night haha."

The Pitch Perfect star decked out his dinner table for the first night of Hanukkah.