'This Is Us' Star Chris Sullivan Is Writing Music With Mandy Moore's Fiance Taylor Goldsmith (Exclusive)

The cast of the hit NBC series is truly growing into an extended family off-screen.

The stars of This Is Us truly are as close off-screen as they are on-screen 

Chris Sullivan, who stars as Toby on the hit NBC drama, sat down with ET's Lauren Zima on Thursday to talk tears, tunes and togetherness.

"The group text is alive and well," Sullivan says of the cast's off-screen bond. "We've kind of settled into our family life and kind of just enjoy showing up for work, doing the job, telling the stories."

What's more, Sullivan says he and co-star Mandy Moore's fiance, Taylor Goldsmith, are collaborating on music.

"Taylor Goldsmith and I hit it off big time, and so we started writing some songs together," the 38-year-old actor says. "I had been writing some songs with another friend of mine, and writing some on my own. And obviously Taylor is one of the world's greatest living songwriters."

Sullivan and his wife, Rachel Reichard, have actually sung together at friends' weddings, "five or six" so far Sullivan says, but noted that he wasn't sure if he'd be invited to sing at Moore and Goldsmith's upcoming nuptials.

"It's up to the bride and groom always," he says.

And, like at weddings, This Is Us audiences always seem to need a tissue handy -- something Sullivan encourages.

"People have to cry. I think inherently the show shows people that there is work to be done in their lives," he says. "There's something about the show they connect with, there's something about the relationships they identify with. And if they're having an emotional reaction to these fictional relationships, then maybe there's something to be investigated there."

And with all the emotions flowing, like his character's struggles with depression, as well as fertility problems with his wife, Kate, played by Chrissy Metz, Sullivan admits that it takes some work to carry the weight home.

"I try to keep myself in a decently light headspace...without interfering with other people's processes or work," Sullivan shares.

"It's a complicated, long process and it's hard to separate real life from these types of scenes," he adds. "It's very difficult to not allow those scenes to affect your real life and put you in a down, depressed, not depressed, down kind of low-key place."

This Is Us does have its triumphant and celebratory moments, but Sullivan isn't giving anything away for what remains of season three.

"I don't know," he says, "but I imagine our writers aren't scared."

This Is Us returns Nov. 13 at 9 p.m. EST on NBC. Check out more about the show in the video below!