There was a moment during the filming of the “Abbott Elementary” and “It’s Always Sunny in Philadelphia” crossover episode that blew Quinta Brunson’s mind. It was the first scene where the core casts of both shows were in the same room.

“The initial moment of all of us on set just rocked me to my core,” said Brunson, star and creator of “Abbott,” in a joint interview with her “Sunny” counterpart, Rob McElhenney. “You have to understand because I’m a fan — it was crazy to see all of you in the school.”

The very idea of putting these two shows together is a somewhat crazy idea that just happens to work. “Sunny” is the profane FXX series that’s set at an Irish dive bar and has been on the air for nearly 20 years. “Abbott” is ABC’s heartwarming breakout hit about the teachers at an underfunded public school, now in its fourth season. But they are both set in Philadelphia, which gave their creators the idea to do an old fashioned crossover, the likes of which used to happen on “Happy Days” and “Laverne and Shirley.”

The first part of the event is scheduled for the midseason return of “Abbott” on Jan. 8. It finds the gang of Paddy’s Pub forced to do community service at Abbott Elementary, a logical explanation for how a bunch of criminals end up around children. It will be followed later by a “Sunny” installment featuring the “Abbott” teachers that concludes the story — the 17th season of “Sunny” recently wrapped production.

Brunson and McElhenney got on a video call with the Los Angeles Times to discuss the process of bringing it all to life.

This conversation has been edited for length and clarity.

Quinta, what was your first experience with “Sunny”?

Quinta Brunson: Even though I was from Philly, I hadn’t actually watched. I came from a very Christian background where that show just could not have been on in my house. So it wasn’t until college — my freshman year in college — and I was in a dorm with my friend Lauren, who is [the person with whom] I discovered all my oddball humor things. We were big into Adult Swim, just the things we weren’t allowed to watch at home. And she was like, “Have you ever watched ‘Always Sunny?’ ” I was like, “You know what, despite being from here, no, I haven’t.” We binged, and it was hard to binge at that time. We’d torrent — sorry — the show from a website nonstop, and I think we watched seasons, at that point, maybe like one through seven. Wait, what are you on now?

Rob McElhenney: 47.

Brunson: No, I didn’t say your age. I said what is the season count.

McElhenney: 17.

Brunson: I couldn’t stop, and I thought it was insane and amazing. It made me proud. It’s how we feel about the Four Seasons. I was like, “I cannot believe this is in my city.” There’s a Four Seasons hotel in Philly that’s incredible. Sorry, just whatever. Anyway, keep going.

McElhenney: We should shill for the Four Seasons because it’s one of the greatest hotels I’ve ever been in.

How did the crossover start to come together?

Brunson: We met at the Emmys. And Rob and the whole “Always Sunny” team had just finished doing a presentation. The theme was TV shows throughout history. They went up there, which was honestly incredible to see. I think you guys made a joke about never having won an Emmy.

McElhenney: Yeah, the premise was TV shows from the past and we were like, “But we’re in the present. Why are we here?”

Brunson: Then I won an Emmy that night, which was really, really cool. And the first people I see backstage was them, and it couldn’t have been more fitting. The first person I saw was Bradley Cooper because he FaceTimed me, and that was right before we were going to film his episode, to say congratulations. Then I run into them, and so I was just having the most Philly [night]. It was so beautiful. I think the Eagles were playing at that time. It just was really gorgeous. Never met them before. Rob and Kaitlin [Olson, who is married to McElhenney and co-stars in “Sunny,”] had told me that they watched “Abbott,” which just made my heart very warm. I think we very quickly said it: “Our shows should cross over one day.”

McElhenney: We continued the conversation at the upfronts. Then it moved quickly past just an aside or a joke, and we started pitching back ideas right there.

Janine (Quinta Brunson), left, and Dee (Kaitlin Olson) in the crossover episode. (Gilles Mingasson/Disney)

“Sunny’s” Charlie Day in one of the “Abbott” classrooms. (Gilles Mingasson/Disney)

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Was it just you two initially in the pitching process?

McElhenney: It was the two of us [and we] just had a basic premise, which would allow us to make both shows and have them feel authentic because obviously they are different styles of show and two different tones. But if we told the same story through two different tones, as seen through the lens of “Abbott,” and then as seen through the lens of “Sunny,” then we could satisfy both audiences. And then because there will be, I’m sure, a lot of people who have never seen “Sunny” before, that will see “Abbott” …

Brunson: And vice versa.

McElhenney: We wanted to make sure that we were making a show that would work for both of them. Once we kind of keyed in on that, it seemed like we could make it work. Then we got the go-ahead from Disney legal, which was a very important part of the process.

And then Charlie [Day] and I went into the “Abbott” [writers’] room and spent the day there.

Brunson: What was nice was they got it and [were] just such giving creators and so willing to get into the world. It was a dream. I’d work with them any day of the week, anytime. When they left, we were like, “That was really nice. It was really fun.” It’s not the most common experience in the world.

Was it always the idea to do one episode of “Abbott” and one of “Sunny”?

McElhenney: I think that’s what we keyed in on very early. That would be the most fun because we get to play the same characters in the tone of “Abbott,” and they get to play the same characters from “Abbott” in the tone of “Sunny.” That’s what will allow us to satisfy the authenticity of each show but then also stretch and do something different.

“Abbott” is a mockumentary so how did that affect how you would play the “Sunny” characters, Rob?

McElhenney: That’s what allowed for us to still be authentic and step into the world of “Abbott” because these characters are going into a school and they’re constantly monitored by cameras, so they would put on an act. If we’re not acting the way that our “Sunny” characters would, it’s because we know we’re being filmed and we’re putting it on the show. We might not use the same language. We might not make our intentions so obvious or known. We might not be wearing our id on our sleeves. Conversely, when they came over to us, we thought it would be fun to see what their characters would be like when the bell rings and the cameras are not on them.

Brunson: Our documentary is being filmed because they’re seeking funds for their school. So you’re going to put, probably, a better version of yourself. Then there are some characters who fit the same in both worlds, like Melissa and Ava, because they’re never really putting on for the camera. I think they’ve done something so masterful, not giving anything away, just with Dee [Olson’s character]. To me, when I first read it, I was like, “This adds another layer to all of this, that if you are a fan of both shows, you are going to have the time of your life.”

Brunson: “Our documentary is being filmed because they’re seeking funds for their school. So you’re going to put, probably, a better version of yourself.” (Marcus Ubungen/Los Angeles Times)

McElhenney: “If we’re not acting the way that our ‘Sunny’ characters would, it’s because we know we’re being filmed and we’re putting it on the show.” (Marcus Ubungen/Los Angeles Times)

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The idea of “Sunny” characters being even remotely near a school is somewhat horrifying. How was that part of your initial discussion?

McElhenney: I feel like we came to that within three minutes of us sitting.

Brunson: I remember we talked about bigger Philadelphia events, right? But it also was: Why are we doing this if we’re not seeing them in the school environment? That’s what really feels fun. When you actually see them lined up in our school, it’s like, “Whoa.” Our show would need volunteers. They would have to be volunteering for the reason that they’re volunteering.

How did you think about matching the different characters?

McElhenney: I wanted to be with Janelle [James, who plays principal Ava]. I said that from the very beginning. I feel like she’s one of the funniest people on television right now. No offense to Quinta.

Brunson: I feel the same way. None taken.

McElhenney: I also feel like her character fits best with what we do on “Sunny.” I think it’s also her form of comedy is my taste as well. But I just feel like she’s so unbelievably funny, and I just wanted to be in a room with her for a few days.

McElhenney said he wanted to be paired with Janelle James, who plays Ava on “Abbott.” “I feel like she’s one of the funniest people on television right now.”

(Gilles Mingasson/Disney)

Brunson: I think the other matches came pretty organically. I didn’t set out to be in scenes with Kaitlin, but when it all panned out, I was like, “Oh my God, I get to do so many scenes with Kaitlin. I think Kaitlin’s incredible. I think she’s one of the most underrated comedic actresses. I think people should talk about her every single day,” and then getting to perform with her, I’d stand by that 10 toes down now. She’s so good. But when I found out I got to be in scenes with her as Dee — I was over the moon. Once again, this is where the “Sunny” stuff comes in handy — remembering that [both characters] went to [the University of Pennsylvania] — it was stuff like that that goes, “Oh my God, this is just naturally turning into something very, very, very good.” I think the other key pairing was Charlie and Barbara, which is probably one of my favorite pairings in the world. That becomes this beating heart, which is sweet because our show does have heart.

And the fact that it still got to have it in this episode in a way that I didn’t even predict. Those scenes moved me. They did.

Did you always know that “Abbott” would air its crossover first?

McElhenney: I think just by nature of the schedule. In some ways, you look at the “Abbott” episode, and it stands on its own, and it’s so great, and it’s a fully realized story. But then when you see our episode, it feels like it’s almost like a giant setup, and then this is the punchline. But then you can watch them in either order, and they both make sense.

Dennis, played by Glenn Howerton, is elusive in the “Abbott” episode. Not to spoil anything, but will things you tease in “Abbott” come to fruition in “Sunny”?

Brunson: Dennis is the key.

McElhenney: Yes. So when you see the “Sunny” episode, you’ll realize why we did that with Dennis.

Was there anything that you were like, “We should save this for the ‘Sunny’ episode. Like ABC standards and practices isn’t going to like this, but FX will be fine with it?”

McElhenney: There’s a joke in your episode that I cannot believe is going to make the final cut. Did I see the final cut?

Brunson: You did.

McElhenney: I cannot believe that that joke was made on your show.

Brunson: I wonder which one.