Andrew Rannells may have starred in “The Book of Mormon,” but now he’s trying Judaism on for size, in the hilarious, heart-wrenching Broadway revival of “Falsettos.”

The musical, with its toe-tapping, Tony Award-winning pop score by William Finn and book by Finn and James Lapine (who directed), is running at the Walter Kerr Theatre through Jan. 8. It tells the tale of Marvin (two-time Tony winner Christian Borle), his boyfriend, Whizzer (Rannells), ex-wife, Trina (Stephanie J. Block), plus Marvin’s young son, therapist and nutty neighbors — an eccentric, extended family of New Yorkers, some Jewish, all neurotic, as tight-knit as they are untraditional. Which is good, because this show tackles some tough subjects (from aging and AIDS to planning a bar mitzvah).

An Omaha native, Rannells, 38, is also known as Lena Dunham’s acerbic gay ex-boyfriend, Elijah, on HBO’s “Girls.” He also appears with Bryan Cranston in the comedy film “Why Him?” premiering in December.

OK — truth — how’s your bald spot?

Ha! Well . . . I don’t have one.

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Mmm . . . I expected Christian Borle to sing that lyric about your character, “I wanna see the bald spot,” but instead he sang about your “hairline.”

Bill Finn rewrote the line for me. He and James [Lapine] looked at the show with fresh eyes, and didn’t hesitate to rewrite lyrics.

I like the way the cast often stays onstage, watching scenes you’re not in.

I love it. Absolutely love it. It’s like we’re eavesdropping. But in the first act, when we all get to sit and watch Stephanie do the song “I’m Breaking Down,” we’re not acting — we’re just sitting there, laughing. She’s brilliant every night. Then in the second act, as reality sinks in for these characters, things become more . . . chilling.

Yeah, when that hospital bed rolled out, a man in front of me burst into tears.

It’s the first piece of real furniture you see in two hours, and all of a sudden — it’s there.

When did you first hear of this show?

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The 1992 Tony Awards — I saw that as a kid. They did the baseball number. The lyrics were so funny. Being a gay kid, seeing gay characters onstage . . . was intriguing. I became obsessed. I went to the library, got the script, and just wore out the CD. It was a bleak time to be a gay child, at the height of the AIDS crisis, wondering, “Does being gay mean you’re going to get sick?” While that’s an element to this story, it also shows you can have a family. It may not look exactly like you thought it would but . . . [I realized] things weren’t quite as bleak as they initially seemed.

That’s a far cry from your “Girls” character. Is there hope for poor ol’ Elijah, by the way, in that show’s final season?

Well . . . I think this is the closest he comes to having some direction. Last season, his heart was broken. This season, he’s still a [expletive], but wiser.

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Was shooting the final season palpably . . . different?

It was. You clock things differently. The final table read, last wardrobe fitting. I think Lena [Dunham], in a classy way, knew when the story was reaching a logical end. We’re not overstaying our welcome. She wrapped things up at a great time.

Yes. Of course, you never know. Bill Finn thought he’d finished telling his story after writing the first act of “Falsettos,” which ran as a one-act Off-Broadway in 1981. Then . . . with the AIDS crisis, he found he had more to say, and wrote what would become act two.

I can’t imagine what it was like doing “Falsettos” at the height of AIDS. To have friends dying around you. Leaving the theater every night and that was your reality. It must’ve been overwhelming. We’re lucky — we’ve made huge advances medically and socially. It’s not the same climate. Although, post-election, who knows what the future’s gonna be? What’s the answer, Joe?

I’m not sure. But it was oddly comforting seeing this show the day after the election.

So many of the lyrics landed in a different way that day.

Like when Stephanie Block sang . . .

“I’m tired of all the happy . . . frightened men who rule the world.” The audience burst into applause. It stopped the show. I mean . . look — it’s hard. But I’ll say this: Knowing that Bill wrote the second act of this show at a time when the Reagan Administration was blatantly denying there was a problem — that Bill used his voice in the best possible way at the time and this beautiful piece of theater came out of it — that gives me a little hope.