Before Jake Gyllenhaal appears in “Sea Wall/A Life” — even before the house lights dim — co-star Tom Sturridge is onstage for a pre-show that consists mainly of him drinking beer.

The brew is real, though Sturridge won’t name the brand.

“Whatever it is,” Gyllenhaal tells The Post, “there are a million bottles of it backstage. I thought to myself, ‘Wait — this is only a nine-week run, right?”

It is. “Sea Wall/A Life” opens on Broadway Thursday, after a brief, acclaimed stop at the Public Theater. The back-to-back monologues by playwrights Simon Stephens and Nick Payne — Sturridge, 33, performs the first, Gyllenhaal, 38, the second — are a roller-coaster ride of emotions: about birth and death, love and loss, and what the “Brokeback Mountain” star calls “the general messiness of life.”

Messy or not, it’s clear the actors feel comfortable with each another. They shot one film together, the Netflix horror flick “Velvet Buzzsaw,” which came out in January, but bonded two years ago, when they first rehearsed these pieces by playwrights they admired.

“My dream version of doing these plays would be to get off the subway, walk into the theater and just do it,” Sturridge says. “Whatever we are [feeling] that day, that’s what the audience gets.”

‘People just want to express themselves.’

Matinee audiences sometimes get even more, in the form of post-show talkbacks. The actors started doing them at the Public — “and that’s the reason we’re here on Broadway,” says the soft-spoken Gyllenhaal, leaning back on a sofa in the Hudson Theatre’s lounge. Sturridge, pale, intense and British, sits several feet away, nodding.

“There are so many things being shared, we feel we’re part of a real community,” Gyllenhaal continues. “People just want to express themselves.”

At a recent matinee, many did just that. One woman said that when her son died 15 years ago, her husband didn’t know how to express his grief.

“She said, ‘I wish he’d seen this [play] when it happened, because you could have been his models,’ ” Gyllenhaal says. “I was brought to tears by that. And then there are other people talking about their babies being born.”

The funniest part of “A Life” is when he voices both a woman in labor and her frantic, floundering husband. Gyllenhaal says his sister, Maggie, a mother of two, gave him a note about that.

“Mags said I could probably do more research on the woman in labor, because I don’t have it 100 percent right,” he says of the actress, who saw the play with her husband, Peter Sarsgaard. “I told her, ‘I don’t think I’ll ever get that 100 percent right!’”

And then, sighing at the question he’s asked so often: Yes, Gyllenhaal does hope to be a father one day. (Sturridge, who has a daughter with former partner Sienna Miller, doesn’t wish to discuss it.)

After such high-voltage performances, the actors say they often unwind over wine, together or with friends. They also have “Tequila Thursdays,” at which actors, producers, crew and ushers gather in the lounge for some post-show tippling.

Next summer, the pair will take the show to London. Do they have plans to team again?

“We’re thinking of making it a franchise,” Gyllenhaal jokes.

Sturridge clears his throat: “Seawall/A Life: Part 2!”