Three decades ago, as one of the Beastie Boys, Adam “Ad-Rock” Horovitz was exhorting for his right to party. In 2015, he’s the voice of adult responsibility — or at least he is in While We’re Young, the new Noah Baumbach film that pulled an impressive $227,000 gross in just four theaters the weekend of March 27 (A24 Films plans to expand the film nationwide April 10). In the movie, Horovitz, 48, plays the rational best friend to Ben Stiller and Naomi Watts, who star as 40-somethings smitten by a younger couple (Adam Driver, Amanda Seyfried) and begin haplessly mimicking their carefree, trendy ways. In recent months, Horovitz also has been working on a Beastie Boys memoir with bandmate Michael “Mike D” Diamond, and figuring out their next musical move.

How did you get involved with the movie?

I know Noah Baumbach from a long time ago. We were hanging out one night, and he asked if I wanted to be in his movie. If somebody whose stuff you really like says, “Hey, you want to do it with me?,” you got to do it. I would like to say that I get these offers all the time, but I don’t.

It’s kind of hilarious to watch a Beastie Boy be the proponent of maturity in the movie.

In a way, I’m the joke. It’s a funny nuance of it — this guy in a band who 25 years ago was known to be this wild and crazy dude and now I’m a grown-up. I’m telling Ben Stiller’s character, “Don’t follow trends — nobody wants to see the old guy at the club.” That’s kind of how I feel in real life: I’m not trying to do whatever is popular. I don’t even know what is popular.

Speaking of what’s ­popular, what do you think about Iggy Azalea? Like the ­Beastie Boys initially were, she has been ­criticized for cultural ­appropriation.

Well, everybody wants to be a rapper — even country musicians. But I don’t know — it’s not like I actually listen to Iggy Azalea, so I can’t really comment. I heard one song and she sounded like a bad version of [1990s female rapper] Da Brat. But for an Australian to want to rap, why is that a bad thing? Like, I don’t give a shit. But that’s me talking. I’ve been all over the world and I’ve seen rappers in Osaka, Japan, in Germany.

Do you and Mike have any musical plans?

We talk about music, but then we’re like, ‘Dude, why are we talking about this? We really have to finish this book.’ So I’m sure we’re gonna do something, but I don’t know what it’s gonna be.

What’s the book going to be like?

Me and Mike have just been writing what happened over the years, to us as a band, as friends and as people, and what New York was like when we were kids. What I’m hoping is it’s not gonna be just about this band where you basically know all the information anyways and there are cool pictures in the middle of the book and that’s it. I want it to be a crazy kind of textbook that’s all over the place.

Are you and Mike still as close as you used to be?

Yeah, we’re definitely as close, we just don’t see each other that much. We used to see each other every day. It’s just proximity-based. He’s kind of all over the place, in California. He’s on the go.

Are you working on music alone?

I work on things here and there. I write some songs with some friends and I’m getting into scoring movies and stuff like that. Tor the past few years I’ve been in my friend’s band, Bridget Everette and the Tender Moments. It’s her thing, so it’s been fun to just be in the background and co-write some songs and play gigs, which I love doing. I make a little music every day just because it’s fun. You know, your basic thing that somebody who only knows how to do music does.

An edited version of this story originally appeared in the April 11 issue of Billboard.