Given her six Tony Awards, two Grammys, an Emmy and a National Medal of Arts, Audra McDonald has nothing left to prove.

Yet there she is in “Frankie & Johnny in the Clair de Lune,” having noisy, simulated sex with Michael Shannon, and walking about, at 48, in her birthday suit.

She looks wonderful, but it begs the question: Why? Why put yourself out there like that?

“You’re always looking to challenge yourself,” she tells The Post in her dressing room, extravagantly eyelashed and outfitted after taping a TV talk show. “The nudity, the graphic sexuality . . . all of it scared me to death!

“Maybe I’m a masochist,” she says, “but I like leaning into things that scare me, because that usually means there’s something to learn.” And since her last show, 2016’s “Shuffle Along,” was a musical, she says she wanted to go in another direction and do a straight play.

The offer to portray Terrence McNally’s wary, weary waitress came “out of the blue,” she says, in an email from the playwright’s producer/husband, Tom Kirdahy.

“Audra,” he wrote, “ ‘Frankie & Johnny’ with Michael Shannon . . . What do you think?”

McDonald didn’t hesitate. She’d starred in McNally’s “Master Class” and “Ragtime,” and admired Shannon for years, ever since he played “the scary boyfriend” of 2002’s Eminem movie, “8 Mile.”

“It was the biggest ‘Yes!’ of my life,” she says, laughing. “Well, one of them.” When Will Swenson asked her to marry him, “I said yes before he even got the question out!”

She says her husband of seven years had no trouble with her performing nude, having bared all himself a decade ago in “Hair.” Swenson’s Mormon family even flew in from Utah to see him.

“They were incredibly supportive,” says McDonald, who grew up in Fresno, Calif., in a family of musicians before coming to New York to study at Juilliard. (These days, she has homes in the city and Westchester County.)

Nevertheless, she says, she was nervous about having her mother see the opening night of “F & J.”

“I told her, ‘If you don’t want to see it, I’ll understand,’ ” McDonald says. “But she came and said, ‘Well, it was just what you’d said it would be.’ ”

The reason she can play the part, McDonald says, is because neither the nudity nor the sex in McNally’s 1987 play is gratuitous. “It’s so necessary to tell the story,” McDonald says. “Once you walk onstage, you’re in it — you’re the character.”

As Edie Falco, who played Frankie on Broadway 17 years ago (and fell in love with her Johnny, Stanley Tucci) told her: “After the first five minutes of the show, you’ll get used to it.”

Then again, the “Sopranos” star hadn’t had a baby 2 ¹/₂ years before, as McDonald had. “I gained a good 60 pounds when I was pregnant with Sally and figured it would come off when it did,” says McDonald, who’s still nursing Sally and whose other child, Zoe, is now 18.

Those pounds are gone. The only “dieting” she says she did was to steer clear from the cookies at the craft table while shooting her TV series “The Good Fight.” (She’ll start filming Season 4 of that show after “F & J” ends its run.)

“I still have that baby belly, but I don’t care,” she says. “These characters are middle-aged, broken people. You can’t play this role and think, ‘Well, I’m going to get as gorgeous and skinny and buff as I can,’ because that’s really missing the point.”

She credits the show’s intimacy coach, Claire Warden, for making sure she and Shannon were comfortable finessing their sex scenes. And an all-female crew — director, lighting chief and stage manager — also made for what McDonald calls “a very safe environment with a lot of communication, a lot of trust.”

If only some members of the audience could be trusted as well.

“Sometimes we see people with binoculars,” McDonald says, “and the cellphone problem is just . . .” She shrugs. “I don’t think anyone’s audacious enough to take pictures with a flash, but we have a great front-of-staff that’s doing everything they can to get the phones or stop them.” For now, she says, one of the biggest challenges is eating that meatloaf sandwich Johnny gives her after they make love.

“It doesn’t have any sugar in it and it’s on gluten-free bread,” McDonald says. “But on a two-show day . . .” She laughs. “It’s not easy!”