[Editor's note: The following contains spoilers through the Season 4 finale of Big Mouth, "The Pornscape."]

Every show evolves as it progresses, but few shows wear that on their sleeves like Big Mouth, the Netflix animated comedy. With the fourth season, now streaming on Netflix, the newly anointed eighth-graders played by series regulars Nick Kroll, John Mulaney, Jessi Klein, Jason Mantzoukas, Jenny Slate, and Ayo Edebiri continue to face old demons like depression and shame, while new foes like anxiety lurch into view. It's a complicated and fascinating new season of television, one which flashes forward into a bleak future dystopia, finds a way to create an unexpected crossover with another show covering similar subject matter, and even spends a little time talking about 9/11.

Collider spoke with co-creators Kroll and Andrew Goldberg about the latest season, including how they approached writing a major casting change into the show after Slate decided to stop voicing the character of Missy, bringing in PEN15 creators Maya Erskine and Anna Konkle for a non-official crossover, and why it's important that their characters never stop growing up. They even teased one potential way that the series could end (though it feels like a long shot).

I want to first ask about the PEN15 not-crossover — what was the story of that coming together?

NICK KROLL: I mean, we love PEN15. I'll be honest, [at first] I was a little like, "Oh, no. It's a show with adults playing kids about puberty." But then we saw it and it's such a special show, it's so funny, and Maya and Anna are such good actors and the show is just brilliantly done. So we then thought it would be really fun to do a subtle crossover where they're not playing their actual characters from PEN15, but they are playing, obviously, seventh-grade girls. And wherever we can have fun little meta experiences like that, we try to do it. We were thinking like Andrew and Nick are now in the eighth grade, and that's now the first time that they're seventh grade girls, and who would be funny to cast in that role.

And we're just such fans of Anna and Maya that we thought that they would be good for it. And they ended up being really perfect for it. And then we added the level that they have their own show "Cafeteria Girls" inside of our show, sort of a tip of the hat to the fact that they literally have their own show about kids going through puberty.

ANDREW GOLDBERG: Yeah, Maya and Anna are so funny, and I remember when they showed up to record, they actually brought their retainers and they were like, "you want our retainers in or out?" We said "In, of course," and they wore them and we were like "you can really hear a difference from your retainers."

And we have Gabe Liebman, who wrote a little bit on Big Mouth, and who had written on Kroll Show, And I think ran Season 1 of PEN15 — someone who's worked on both of our shows and we love and think incredibly highly of. We're just such a big fan of theirs — it's one of the great things about doing our show is you can really be fans of people because it's animation, they could actually be a part of it, and they oftentimes will figure out how to make that happen. This season is filled with those people, like obviously Seth Rogen, and John Oliver, and Maria Bamford, and Josie Totah, like all of these different people who came to be part of the show — Sterling K. Brown, we got Paul Giamatti to be a sentient piece of poop...

So when you're creating the character of a sentient piece of poop, are you thinking, you know who would be great for this is Academy Award nominee Paul Giamatti?

GOLDBERG: Yeah. When we're writing it, it often helps us think of a person. And sometimes it's like, well, the perfect person would be so and so, you go out to them first, and sometimes they say yes.

So was Paul Giamatti your first choice for the sentient piece of poop?

GOLDBERG: Of course, how can he not be? And I believe his kids... Were his kids fans of the show? I think his son was a fan.

When it comes to the voice casting, let's talk about the transition of having Ayo [Edebiri] take over the role of Missy. In terms of this timeline of events, at what point did you know that it would happen in that episode? Because I imagine that the structure of Episode 9 was already really in place — at what point did you know that would be the episode you'd want that to happen?

GOLDBERG: Yeah, by the time Jenny decided to step aside and we cast Ayo, we had already delivered all of Season 4 of the show and we knew that it would be impossible to go back and replace all of that dialogue. And two, that it wouldn't really be great for Ayo as the new actor taking over the part to have to match Jenny word for word the whole season. And then Jen Flack, who's one of our partners who created the show with us, had the idea of doing the transition during that Halloween story, because there already was this magical moment that was on theme, about Missy putting together all the different parts of her identity and the shattered years, and the shattered years forming this mosaic Missy. Jen had the idea that Ayo would be the voice of mosaic Missy, and then after that, that's when the change would take place — in the moment where Missy really put together all the pieces of her identity. So Ayo voices Missy for the rest of that episode, and then for the last episode of Season 4, and then she'll be Missy from Season 5 moving forward.

Great. So the mosaic idea was already there — that was just a good transition point?

GOLDBERG: Yeah. We were really fortunate that we were already dealing with this theme of Missy's identity and in a way that allowed us to figure out a creative, organic way to make the handoff from actor to actor.

Wonderful. What about Ayo's performance that made you feel like she was the right person to take on Missy?

KROLL: Well, I've known Ayo for a few years through stand up and we had her join our writers' room for Season 5, because we went well ahead of what is coming out. So we had really gotten to know her. And we auditioned a ton of people, we even looked into Twitter and Instagram people who had been in stuff, and obviously through all the regular casting channels. Ayo auditioned along with everybody else, and we just thought she understood that Missy, both like how the voice to transition, but also like she really understands Missy on a deep intrinsic level. I mean, she would, I think, self-describe as a Missy. Growing up when you see a picture of her at 13 in suburban Massachusetts, you are like, "Oh, that's Missy." So I think she just vocally understood how to keep that transition smooth, but also make it her own and on the really deep, emotional and creative work, she really understands deeply the character. And so it was a very natural fit for her to come in.

There's a big episode this season that involves 9/11, and I wanted to ask — what kind of approach to comedy surrounding 9/11 makes sense to you?

GOLDBERG: Well, I mean, I think for us, one of the things that was interesting to us was the realization of how young our characters were when 9/11 happened or rather that our characters weren't alive when 9/11 happened, whereas we're all adults and you can actually remember it. So it would be interesting to tell a story with these characters who don't have that personal connection to 9/11.

KROLL: It's such a weird thing for kids right now. Kids who were graduating high school this past year were born the year after 9/11 and are graduating high school the year that this global pandemic has happened. It's quite a bookmark for a childhood. That, apropos of nothing, is just something that I've been thinking about.

It's a heavy thing. You're of course are writing well ahead — is the pandemic something you feel like you have to include in future seasons?

GOLDBERG: Well, with our animation process, we're so far ahead with writing, we wrote and recorded Season 4 last year. And we knew that it was going to come out generally sometime around the election, and that anxiety would be the emotion of the moment. We obviously didn't know that there was going to be a global pandemic, but I think when we're choosing these scenes whether there's shame or anxiety, we're always looking for something that is timely, but in some way timeless.

And I think that's what anxiety is. I think one of the reasons we might've done anxiety is because we have a few groups of teenagers that we talk to every season and we run ideas past them. We get a sense of what things are important to them and what their lives are like. And one thing that kept coming back was anxiety, whether it's anxiety about school or about their parents or about their safety or about fitting in with your friends. And I think that really was true, the anxiety, we just didn't know how much it would be the emotion of the year.

That's definitely true. And the fact that climate change is a big factor in the season as well speaks to that.

GOLDBERG: Yeah. I think that's one of the factors of anxiety for kids, because they know that they're going to be here longer than adults.

I've got one last question, which is just essentially — do you have an end game in mind for the series? Like, do you have a sense of where you want it to wrap up?

KROLL: I think with Nick and Andrew, creating an animated show together, about their experience as children. So we're just going to keep going until they catch up to us.

Is that actually legit the end of Season 6?

KROLL: No, I mean, yeah, it would be amazing if we could see them through Season 5, through somewhere in eighth grade, and then Season 6 was about 14 to 37. I don't think we know. The beauty of Netflix having picked us up through Season 6 was it gave us the time to sort of map out our stories, to give us breathing room to know we have time to tell the stories we wanted to tell. And what's weird and really enjoyable about our show is that it's about kids going through puberty and literally going through changes, and so we get to continue to evolve. Our kids continue to evolve and change and grow both emotionally and physically.

And I think we're going to continue to do that. Andrew could say otherwise, I don't think we have an exact idea of when we're going to end the show or how we're going to end it yet. We just know that we're going to keep telling these stories, as long as we feel like we have more interesting stories to tell. I still love making the show, I think Andrew does too, and I think our team really loves making the show, and so we're just going to do that and see where it takes us.

GOLDBERG: You change and grow actually in the show of longevity. And I remember like for me, that idea took some getting used to because I really grew up on stuff like The Simpsons, and those shows where the characters didn't really change and they didn't get old. But Nick and some of our other writers were rightly so adamant. They argued like, "No, our show is about growing up and changing, the characters have to grow up and change." And I love that that happens now, and I think that's one of the reasons that we don't run out of stories, because our characters do evolve.

Yeah. I did love the Simpsons joke in this season.

GOLDBERG: Yeah, and it's beautiful and it works for them. But our show is different, which is, I think one of the really great things about animation right now. There are all different kinds of shows.

Big Mouth Season 4 is streaming now.