Family dysfunction and thousands of gallons of water in the theater course through the play “If There Is I Haven’t Found It Yet,” now in previews for a Sept. 20 opening.

Written by young British author Nick Payne, the dark comedy is noteworthy for marking the New York stage debut of Hollywood heartthrob Jake Gyllenhaal.

The “Brokeback Mountain” Oscar nominee plays errant Uncle Terry. Oklahoma-raised Annie Funke is his tormented, overweight 15-year-old niece Anna. Her parents are played by Irish actor Brian F. O’Byrne, a Tony winner for “Frozen” also known from TV’s “Prime Suspect,” and Scottish actress Michelle Gomez, another Off-Broadway newcomer.

Over coffee and some gentle ribbing, the four talked about being a family — on and off stage.

Daily News: Jake, what drew you to this play to make your American stage debut?

Jake Gyllenhaal: I read it a few years ago and fell in love with Nick’s dialogue. I think any story about a family is fascinating, too, because it’s filled with complications both universal and specific. There’s never-ending inspiration in that. If you’re going to do a show eight times a week, you need to have stuff to draw on every night.

DN: Brian, you play George, a man obsessed with the environment, but oblivious to his family’s woes . Why’d you sign on?

Brian F. O’Byrne: I haven’t done a play in five years, not since “Coast of Utopia.” This play arrived and it’s a fascinating subject matter. Michael Longhurst, our director, has brought in a bunch of scientists and therapists in family issues who’ve come in and spoken to us. It’s made the process very interesting.

DN: Annie, the play begins with Anna in the aftermath of getting bullied. Can you relate?

Annie Funke: When I read the play, there were a lot of things I identified with, things from my own background. I was bullied — mostly about my weight. In middle school, I was called “Chunky Funky” a lot. Girls would sing the Jenny Craig song to me. It brought up things I’d pushed away and hadn’t thought about for a very long time.

DN: She’s not the peppy Tracy Turnblad of “Hairspray,” who you’ve played.

Funke: It’s something completely different from what I’ve ever played before. I always play the bubbly, excited, leader types. But doing a play was something I’ve always wanted to do — and one that might actually be more than entertaining. Some bigger girls have come up to me after the show and thanked me for telling their story and making them feel less alone. That’s a real honor.

DN: Michelle, you play Fiona, a working mom with her hands full.

Michelle Gomez: Right. I feel like I am Fiona. There’s a line where I’m asked, “How is everything?” and I say, “Busy.” I just bought a New York apartment, am opening a play and I have a 2-year-old son. Like Fiona, I’m a mother spread too thin. And it’s all being channeled into this performance, for sure.

DN: Jake, you’re an uncle to your sister Maggie’s two young kids. Has that informed your work here?

Gyllenhaal: Of course. I think real life is bound to affect any sort of artistic process. I have two beautiful nieces and they’re both an inspiration to me. One of the things that I love about this play is that it’s about a relationship that I understand.

DN: And hopefully you’re less inappropriate than Terry, who has a knack for poor decisions.

Gyllenhaal: I don’t have a similar experience to the one that we have in this play, but being an uncle is something I draw up on every day and night. The family is a microcosm for political themes that are involved in this play — the environment and the issues going on of how we treat the world.

DN: It seems to riff on “think globally, act locally.” If you get your house in order, the ripple effects are huge, yet?

O’Byrne: Yes, I think so. George seems to be tripping on his family, looking towards something he thinks he sees in the distance. Stepping all over them, he can’t hear their cries .

Gomez: It feels we have a chance to impact people with a message: Clean your own house and that will have a domino effect. That has never been more relevant than it is today. Our biggest enemy with society is inertia. As soon as that sets in, it’s a poison .

DN: I’ve heard that water plays a significant role in the Roundabout production — and that there’s 3,750 gallons of it. There’s even a “splash zone.” Without spoiling things, what’s up with that?

Gyllenhaal: When Michael pitched to us what he was going to do with water in this show, Br ian’s face changed — his eyes got wider than I’d ever seen before. He may have wanted to walk out at that moment. [O’Byrne nods, and laughs.] The water is a beautiful metaphor. I really think that Nick and Mike are a new wave of British talent and this production a perfect example of how to work together. This production is very different from the one in London a couple years ago.

Gomez: The water takes it to a place that’s other-worldly, actually. Just make sure you’ve peed before you sit down.