It was the late 1990s, and Spencer Liff was living a double life. By day, he practiced his jetés and soft-shoe at the American Ballet Theatre and the Steps on Broadway dance studio. By night, he was sneaking into clubs like Tunnel, getting a very different kind of dance education.

“I remember seeing these larger-than-life people,” the now-33-year-old choreographer tells The Post. “The way they danced and moved was the pinnacle of self-expression … And I’d just never seen that on a Broadway stage.”

Now he’s put it there. Opening Thursday, July 26, “Head Over Heels,” featuring music by the ’80s girl group the Go-Go’s, has all the vogueing, tutting and waacking Liff saw while exploring New York’s underground.

Although Liff has created moves for other shows, including “Spring Awakening,” the West Village resident calls “Head Over Heels” his most personal effort to date: “It has my favorite elements of dancing, and of growing up in New York, all mashed together.”

He first read the script four years ago, while teaching Neil Patrick Harris to sashay in platform heels for “Hedwig and the Angry Inch.”

“I knew exactly how these people should look and dance and move,” he says of the characters in “Head Over Heels.” “I called my manager and said, ‘This show is the one that’s going to change my life.’”

“Head Over Heels,” about a king whose world is turned upside down by a gender-fluid oracle, opens with what looks like a still-life: people in Elizabethan garb gathered around a table laden with wine and food. But as “We Got the Beat” starts up, they strike angular poses and engage in a synchronized kind-of playground clap.

“A lot of the choreography in that opening number is meant to be incredibly precise and intricate,” Liff says. “Militaristic, but still sassy, fun, kooky, queer.”

The vogueing poses repeat throughout, mixed with Bob Fosse-style jazz steps and balletic pas de deux. One Act 2 number, to the sugary rocker “Turn to You,” even incorporates elements of traditional Elizabethan court dance — but with same-sex couplings.

“I decided that the most traditional style of dancing would only be done in the most nontraditional sense,” says Liff, who hopes theatergoers take its celebration of diversity and tolerance to heart.

“No matter what happens with this show, whether we’re a big hit or close immediately, I will be so proud,” says Liff. “People have been calling us the gayest show that’s ever been. But we’re not a gay show, we’re a queer show — we’re all-encompassing, and I embrace that.”