It’s good to have an alter ego, as comedians Nick Kroll and John Mulaney know quite well. They’ve been playing crotchety, 70-something Upper West Siders Gil Faizon and George St. Geegland for a decade — first at a stand-up gig and on Kroll’s Comedy Central series “Kroll Show,” then Off-Broadway, on a national tour. Now — and they’re more shocked by this than anybody — they’ve brought their characters to the Lyceum Theatre in “Oh, Hello on Broadway.”

Or “Br’d-WAYYY,” as the two old codgers pronounce it. Running through Jan. 8, the experience is part stand-up, part farce, part talk show (during each performance they call someone up onstage, often a celebrity, from Seth Meyers to Cuba Gooding Jr.). And the audience, made up of “comedy nerds, theater dorks and children whose parents have made a severe mistake,” as Gil describes it, roars.

Newsday contributor Joseph V. Amodio recently chatted with Kroll, 38, and Mulaney, 34, on a conference call.

Excuse me if I sound distracted. It’s just that there’s a leak in my apartment. As we speak, water is dripping down my bathroom wall from somewhere above so . . .

Kroll: Full disclosure, that’s George and Gil. They clog up toilets all over the city.

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Mulaney: They try to flush the Sunday Times every weekend.

I shoulda known. By the way, I love the inspiration for these two characters — how you saw two older gents at The Strand bookstore downtown buying Alan Alda’s autobiography and they . . . really captured your attention.

Mulaney: We were looking for a way to host a stand-up show and they were . . . a perfect pair. We could tell they’d been joined at the hip for years.

Do you do that a lot — see people from afar and imagine their back stories?

Mulaney: We had more time to do that back then. Now we get lots of photos every day on Twitter from people . . . spotting other Georges and Gils around the country.

Kroll: They’re everywhere.

Mulaney: There are female Georges and Gils, too.

Kroll: But the show is also about Broadway, and New York. We’re history buffs, and love the city of the ’60s, ’70s and ’80s. And Ed Koch . . .

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Mulaney: It was a time when sweaty politicians yelled at reporters and reporters yelled back. I pine for that now a little.

The Long Island Rail Road even gets a shout-out in the show. George St. Geegland’s novel about commuters is entitled “Next Stop, Ronkonkoma.”

Mulaney: I don’t think you can pass by the Ronkonkoma stop more than once without the name leaving a lasting impression. It sounds like an ancient tribe of . . .

Kroll: . . . of Islanders fans.

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Mulaney: Yeah . . . that went to Adelphi.

You two met as students at Georgetown. It’s hard to imagine you hangin’ with the Jesuits but . . . OK.

Kroll: There wasn’t a ton of focus on the arts there, but, because of that, we had a ton of opportunities to get onstage.

Mulaney: We met when I was a freshman and auditioned for the improv group. Nick was a senior and the director.

Kroll: And I’ve been holding that power over him ever since.

Does doing the show on Broadway feel . . . different?

Mulaney: It’s very rare air to be in. We started rehearsals at a studio with other Broadway shows down the hall . . . like “The Front Page.” It was flattering.

Kroll: Everything’s on a grander scale now. Our sets and . . .

The massive tuna-sandwich puppet.

Kroll: You have to spend a lotta money to see these shows. We understand that. We wanted to make it worthwhile.

You sometimes invite celebrities onstage. Who have you really clicked with?

Mulaney: It was great having Alan Alda opening night.

Kroll: He was so funny and sharp. We don’t plan anything. It’s a blast, getting to talk with these legends. Like Whoopi Goldberg, who did her show at the Lyceum years ago. Or Stephen Colbert.

Mulaney: Former Police Commissioner Ray Kelly was surprising. We thought we’d have to prep him on the characters. But he’s the former police commissioner . . . so he was like, “I know exactly who you guys are and what to do.”

Kroll: Aziz Ansari came on with his dad. We didn’t have a guest one night and Katie Couric was in the crowd so we pulled her onstage. You just never know. Our goal is to make every show a new experience.

Has doing the show made you think about what you’ll be like in your 70s?

Mulaney: A lot. On two-show days, we’re in makeup for, like, seven hours. You have to stare at yourself as an old man in the mirror . . . which is a good exercise. More people should do it. Makes you want to eat better. And have fewer regrets.

Kroll: Yeah, hopefully, we’ll have taken better care of our bodies than George and Gil have.