Almost 12 years ago, actor Michael Cera sat down between two poorly-lit ferns and fielded awkwardly aggressive interview questions from comedian Zach Galifianakis. That was the very first episode of Between Two Ferns, intended to be a one-time sketch for Will Ferrell and Adam McKay’s website Funny or Die. Since then, the series has accumulated 21 episodes, earned two Emmys, and interviewed a sitting president (Barack Obama, in 2014). Then, after a special on-the-road episode for Comedy Central in 2012, “Between Two Ferns: A Fairytale of New York,” co-creator Scott Aukerman and Galifianakis got an idea: Why not try for a Between Two Ferns feature film?

This Friday, September 20, that idea will become a reality with Between Two Ferns: The Movie on Netflix. Following in the tradition of mockumentaries like This Is Spinal Tap, the majority of Between Two Ferns: The Movie is improv. Aukerman, who is also known for his comedy podcast and TV show Comedy Bang! Bang!, had no script, just a detailed outline. Or, as he called it when he spoke on the phone with Decider, “a rambling manifesto.” The plot is as simple as it gets: After Galifianakis accidentally kills Matthew McConaughey in a freak piping accident (OK, maybe it’s not that simple) Ferell—portrayed as an evil Hollywood overlord obsessed with his website’s “clicks”—orders him to deliver ten more Between Two Ferns celebrity interviews. If Galifianakis can do that—with the help of his trusty crew, played by Lauren Lapkus, Ryan Gaul, and Jiavani Linayo—then Ferell will grant him his very own late-night talk show. Inexplicably, these celebrities are scattered across the country rather than being based in Los Angeles—but the film is so ridiculous and hilarious, you don’t question it.

The real focus, of course, is the many, many celebrities that appear in the film, which include Chrissy Teigen, John Legend, Hailee Steinfeld, David Letterman, Paul Rudd, Jon Hamm, and many, many more. And no, they don’t know Galifianakis’s questions ahead of time, and yes—as you’ll see in the first-ever Between Two Ferns blooper reel—they are in on the joke. “Honestly, these celebrities are better at staying in character than Zach is,” Aukerman said.

Decider: How do you direct a movie without a script?

Scott Aukerman: It’s really difficult to do. I think that our crew, especially, was looking at me like I was an insane person a lot of the time. The script just looks like a rambling manifesto—just ideas written down and no dialogue. I would try to include the location and what it sort of looked like—stuff that they needed to do their jobs. It was a really unique way to do a movie and something kind of everyone had a little trepidation about. We tried to do it like an actual documentary: A bunch of ideas for scenes, and locations. I was often inspired by the locations more than just trying to figure out a plot. For instance, our location manager took a picture of the lake with all the swan boats in it. We were already shooting at this point, but I was like, “Yes! I like that, I’ll write a scene for that.” That’s what happened throughout the whole movie. Most of the movie is shot in LA, but we did go to New York to shoot a bunch of it as well.

Was there a certain number of celebrities you wanted to hit to make this feel like a Between Two Ferns feature film?

No celebrities were booked when we started, which is a semi-terrifying way to do a movie. I’m pretty sure Tiffany Haddish was the first one to jump on board—she came on, I think, the third day. But we just would slowly slot celebrities into the schedule, and I was used to it, because I did a television show, a fake talk show, for five years that was shot the exact same way, called Comedy Bang Bang. I said, “If we can get 14 celebrities to do [Between Two Ferns: The Movie], then I think that we’ll be covered.” Then we just kept getting people to agree to do it, so I kept adding people, writing scenes for them. It ended up being like 20 really big celebrities who ended up agreeing to be in it.

Did anyone not make the cut?

No. But I will say, I’m really good friends with Adam Scott and we filmed an entire interview episode with him that we ended up not using. I had to break it to him that he’s barely in the movie. The first cut of the movie was three hours long, and I had to get it down to 82 minutes. The reason was that it tied into a plotline that we ended up discarding: The original ending. Zach goes back to the public access station and buys the station with the money that made on one episode of his Hollywood talk show. He rebuilds it, but he doesn’t do it up to code, because he wants to save time and money. Then there’s this tap-dancing show happening above them and Mary Holland—who’s a great improviser—had a big scene where she and Zach are arguing about the tap-dancing show interfering with Zach’s show. Then there’s this giant stunt where the roof caves in and the tap dancer comes crashing through during the interview. We filmed it all. Then we filmed Zach and Adam crashing through the floor as well. Majorly expensive stunts that we ended up not using.

Oh, wow. That’s gotta be on the DVD—except it’s a Netflix movie.

Yeah, exactly. I think we’re going to put out some of the Adam stuff somewhere, so watch for it.

Not to ruin the magic, but I’m curious: Before the guests sit down for an interview with Zach, what do you say to them?

I have a pre-planned speech that I give to people. I found, earlier than the movie with a few of the interviews, that I can’t take it for granted these celebrities even know what they’re doing there. A lot of the time, celebrities are just kind like shoveled into rooms and told, “Hey this is a thing,” but they don’t even know what it is. I would give every person the pre-planned speech: “Hey, so this is an interview. We’re not gonna show you the questions. Zach is going to be mean to you, but that’s part of the fun.” Normally—like, Keanu Reeves at this point, interrupts and says, “Yeah, I know all this. Everything is cool, I get the joke, don’t worry.” But I would continue on if people still looked confused. Honestly, these celebrities are better at staying in character than Zach is. Zach breaks all the time, feels bad about asking mean questions, and apologizes. I’m the one who’s normally saying, “Zach! Stay in it stay in it!” The celebrities would rather he not break character all the time as well.

Do you ever set up ground rules, or allow a publicist to say what’s off-limits?

We want everyone to have a good experience with it so usually what we’ll say is, “If, in the moment, it strikes you as being too off-color or something that you don’t want to talk about, then just call it out to us in the moment. We’ll honor that and not put it into the edit.” That’s happened to us several times over the years. Usually, I don’t even like to talk about examples of it, but Zach’s talked about this so I think it’s fine—for the Jennifer Aniston one, we had hired a Brad Pitt look-alike who was going to come in. At the last minute, Zach chickened out and asked Jennifer Aniston if it was okay if we did that. He just felt too bad about springing it on her.

She very nicely—and I agree with why she said this—she was like, “Uh, I’d rather stay away from it. I’m just so tired of talking about the Brad Pitt stuff, and anytime you bring it up, it prolongs the conversation.” Zach had to break it to the Brad Pitt impersonator that we weren’t going to use him, and the guy was bummed. Zach’s like, “Well you know maybe we’ll call you for something else. Do you look like anyone else?” The guy was like, “Colin Ferrell?” So if we ever do a Colin Ferrell episode, we have him in the chamber.

You’ve said the one interview you really wanted to get was David Letterman, and you got him. How did you feel about roasting your comedy hero?

Zach, at least, had been on his show, though that doesn’t necessarily qualify as having a real interaction with him. You hear stories about David Letterman being cantankerous and not suffering fools gladly, so we were really intimidated by it. He couldn’t have been nicer about the whole thing. He complimented us and the show—he had watched a whole bunch of episodes the night before and he was talking to us about his favorite jokes. He would laugh at all of Zach’s jokes. I was just trying to not be the nerd who bothered him by saying, “Hi you’re one of the reasons I got into comedy.” I never said that to him, but I had the set photographer keep surreptitiously snapping pictures of us together. I made him laugh, so that was a big thrill for me.

The blooper reel was great, but this is the first time we’ve seen behind-the-scenes footage for Between Two Ferns. Why did you decide to include it?

It was a really last-minute thing. It was an idea that Zach had for a while that I was resistant to. I really didn’t want to puncture the reality of the movie. I mean, movies have had blooper reels all the time: Smokey and the Bandit, The House starring Will Ferrell. But we’ve been doing this for 10 years and in the early years, we had a rule that none of us would ever do interviews about the show. Rolling Stone called us and wanted to do an article about the show, and we said, “No.” We wanted people to just enjoy them, and not feel like they were pre-planned.

At a certain point, we stopped caring about that and gave interviews. Zach wouldn’t do a lot of press for it, but I did press for the Obama one. But I still was resistant to it, because I didn’t want anyone to necessarily see that these weren’t technically as uncomfortable as they are on screen. But Zach really pushed for it. We said, “OK, we’ll try it.” We put it up in front of an audience, and they just loved it. They erupted. So I admit I was totally wrong on it.

But to me, it also seemed like Zach was trying to say, “We’ll show the behind-the-scenes because we’re not going to do Between Two Ferns anymore.” I was trying to say, “Come on, we can still do more of these.” The movie was so hard to do, that at a certain point we were tired of shooting it, and he was saying, “Nope, this is it.” Then we took a break, I didn’t see him for a couple of months, and when I saw him the other day he was like, “Hey, what about a sequel?” [Laughs.] I think we needed a little time away from each other.

So it sounds like a sequel is on the table. Do you have a title and plot in mind?

The title we have in mind is Between Two Ferns 2: Between Three Ferns. So that will be the sequel. We had an original idea for the end of this movie that, very early on, we ended up abandoning. I think we’d love to do it for the second one if we can make it happen. But it’s such a giant idea, I don’t wanna spoil. It definitely is something that, if we could achieve it, think it would really elevate the next one.

Can you give me a hint?

I’ll give you the location: Earth.

Pretty limited then, if you’re not going to space.

It’s kind of limited, yeah. We’re not gonna go to another planet. As of this moment, the scientists have not discovered life on other planets, which is a bummer. I mean, they barely have found water on Mars. We’ve been to Mars so many times, and they just found water? It’s like, come on, you dumb NASA nerds. This is, by the way, coming out on September 20, when everyone is gonna storm Area 51. So if there are aliens then I would mobilize a crew and get down there. The first interview with an alien—I think has to be Zach Galifianakis.

This interview has been edited and condensed for clarity.

Watch Between Two Ferns: The Movie on Netflix