It has been 15 years, and we still can’t look at a bedroom door the same way.

History credits Toy Story with blasting open the gates for success over at Pixar Animation Studios, and that’s a perfectly deserved attribution. But the studio’s fourth feature film, Monsters, Inc., which celebrates its 15th anniversary on Nov. 2, proved something that’s perhaps forgotten in the cinema studies books: that the Pixar effect wasn’t just a fluke of the directorial guidance from Pixar chief John Lasseter, but something that eked its way into what would grow to become a roster of some of the finest directors in animation.

Enter Pete Docter, who had the intimidating challenge of being the first director of a Pixar film after Lasseter helmed the first three. Taking up the mantle was a daunting task, to be sure, but Docter (with co-directors Lee Unkrich and David Silverman) turned a trial into a testimony of the success of what could be possible from the little studio up in northern California. Monsters, Inc. was critically acclaimed (it stands at 96 percent on Rotten Tomatoes), pulled in over $577 million worldwide after its 2001 release, and went on to spawn a 2013 prequel, Monsters University, that would earn even more.

EW sought Docter (who went on to write and direct Up and Inside Out) to look back on the development of the film, the early choices that helped crack the core character quartet, and the enduring legacy of a bold original idea about the real story behind the monsters hiding under your bed.

ENTERTAINMENT WEEKLY: 15 years! How does that sound?

PETE DOCTER: It sounds totally weird. It doesn’t seem like that long ago that it came out!

What moment did you see this film taking on a bigger shape beyond just becoming a nice success at the box office? Beyond numbers?

For me personally, I think that was just maybe last year. [Laughs.] At the time… I thought it was going to be a disaster and it wasn’t. People liked it and it [was] well-reviewed, and that was very gratifying because it was a heck of a long road, and very difficult for me personally. That was the first film that Pixar had done that was not directed by John [Lasseter], so I was really having to follow in his footsteps and not be overshadowed by the way John likes to work, because I was trying to find my own way. That was a whole very difficult road. So then when it came out, that was great and I felt like, okay, I didn’t screw up.

But now, recently, people come up to me, and they’re usually around 20, and they kind of say, ‘Monsters, Inc. was the movie that I grew up with.’ And I recognized that look from myself, having grown up with the films of Disney and the Muppets, and meeting Frank Oz and Dave Goelz for the first time. Those were guys that formed my sense of what entertainment is and who characters are, and now I can kind of see it in their eyes. So that’s really weird.

What do people ask you the most?

A lot of times people ask about the recording sessions, specifically for the kid, because in Monsters, the voice of Boo was actually one of the story artists’ daughters. She was too young at the beginning. The reason we got her was because she was on the cusp of language, so our thought was, when kids babble, it might sound like a different language that monsters don’t understand. But then it just ended up sounding like a cute kid and we really liked that. The challenge in working with her was it was not like working with an actor, because she didn’t want to do what you asked her to do.

I think Billy Crystal’s Mike Wazowski will likely go down in voiceover history with enough years under the belt. Tell me a little about the first time you saw him in the booth.

We had recorded — there was a guy here named Jeff Pidgeon who was doing scratch — and then you walk in and there’s Billy Crystal, oh my gosh. When we first talked about it… you probably know the whole backstory of how he was approached to be the voice of Buzz Lightyear, right? So John and I went and pitched him [for Monsters] at his office. I kind of got the sense that he probably would have said yes even if we hadn’t pitched him anything, just because he was a fan of the films we did. But then when we first recorded, I remember him saying, ‘Oh, what kind of voice does he have?’ And I thought, ‘Uh oh.’ Because here’s what usually happens: We get actors who think, it’s animation and I can do a silly voice! And then it doesn’t sound truthful, it doesn’t sound real. Of course, Billy is a comedian — he can do that. But even then I was a little nervous because we had thought of him in terms of his natural voice. So where we ended up [with Mike’s voice] was him turned up a little bit. It’s him, Billy, slightly higher, slightly more caricatured in terms of an east coast accent. Just workshopping, that was the first order of the day.

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Let me give you one more quick thing about Billy: He, as a brilliant comedic mind, would play around with stuff all the time, so any recording session would almost be like a free stand-up comedy performance. We would be laughing hysterically. There was one time when, by accident, Doc, the recording engineer, had plugged in a reverb and he’s like, ‘Oh, I’m sorry, I’ll turn that off.’ And Billy’s like, ‘No no no, wait wait wait.’ And he did a whole kind of [run] as though he’s an announcer at a baseball game, just off the cuff, and we were covering our mouths just to not laugh and ruin the whole take. It didn’t end up in the film but it was just fun. He’s just such an amazing mind. He gave us so much more than just his voice.

Was John Goodman a similar story? I feel like his performance gets lost in the shuffle. There’s tenderness and humor there, and it’s really a tremendous vocal performance.

Yeah. He was less like, ‘Hey, I want to play around with this’ and more like, ‘What do you have here and how do I work with what you’ve got?’ But he’s the emotional heart of the thing. He’s just such an amazingly instinctive actor. He stepped right in and he nailed it.

It seems to me that Billy and John — and, yes, Tim Allen and Tom Hanks as well — unlocked what you guys could do with an A-list pair.

This is going to sound weird, but that was the first film that we worked on with multiple characters in the booth at the same time. On Toy Story, we had sort of, I guess initially for schedule reasons, had not worked with Tom Hanks and Tim Allen at the same time. But with Billy and John, the energy level would go up. They would find entertainment in trying to crack each other up. That whole scene where they’re in the locker room at the end of the day and they’re like, ‘Hey, do you have any odorant?’ ‘Yeah, I’ve got wet dog and…’ They just improved a lot of stuff. They were really trying to push it. And I think having the both of them there in the studio really helped the believability of these two best friends.

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Let’s dive into some characters. Sulley went through literally dozens of core changes, but what was the benchmark epiphany with the character?

I think the trick with Sulley was, we came into the project thinking there’s no way the audience will empathize with a monster who scares children. That just seems mean and unappealing. And so we bent over backwards trying to create a story that still had monsters but Sulley was on the sidelines. He was a janitor, he was a failed scarer, he was all these different things — and when we finally let that go and just said, “Okay, he’s the BEST scarer there. He’s the star quarterback,” then it all came into focus. Because until then, design after design, we really didn’t know what he was about.

I think another thing for him was, we had been warned early on not [to] make this look like a guy in a suit. Maybe ‘warned’ is the wrong word, but it was (then-Disney Feature Animation president) Tom Schumacher who gave us that feedback as we were starting the design. We don’t want this to look like a man in a costume, and we thought that makes total sense. So that’s why we went with tentacles and played around with proportions, all this kind of stuff trying to push it away from that. In the end, that proved to be both technically challenging but also aesthetically challenging — to have like, five eyes, it’s kind of off-putting. So that’s why we ended up…he’s the star quarterback. He’s big, strong, lumbering, but still sort of lovable. That’s why I really wanted the fur, which was a huge challenge for us at the time.

Is fur innately the most lovable texture?

Yeah. It’s why we even like pet rats more than spiders and snakes. In general. I mean, some people like those, too — I don’t want to say anything negative about pet snakes. [Laughs.]

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When I was covering Finding Dory, everyone would often cite Monsters, Inc. as trailblazing in experiments with tentacles. Can you imagine going back to Monsters, Inc. now with the technology Pixar’s developed in the 15 years since?

Oh, yeah. Even from Monsters to Monsters University, I would go into reviews and think, ‘Oh my gosh, you guys are so lucky.’ We were not physically able to render more than one monster with fur on the screen at a time. We had to really camouflage. If we had multiple monsters, we’d have to figure out how to break it out into separate layers. We were very limited with what we could do with the complexity of fur, it was just too much to handle.

Wait, didn’t Sulley appear at one point with…what’s the name of the monster with the sock on him? The quarantine fella?

Oh, George Sanderson! [Laughs.] Yeah, I don’t remember if there are any shots or not with the two of them. There must be, somewhere.

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Talk to me about Mike’s evolution — he wasn’t even part of that first pitch.

Mike was not in the first draft. Our first movie was about Sulley and this kid, and then in subsequent story note sessions, we thought it would be nice for Sulley to have a little bit more of a backstory and a world that gets upset. So we came up with [this] random background character that Ricky Nierva had drawn. He was one of our storyboard artists who later went into the art department. But we kind of went back to this drawing and said, there’s something about that guy.

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He definitely looks like a monster and being that he’s going to be not THE main character [but] kind of a sidekick, we can push some of the stuff like only [having] one eye. Initially we had experimented with only giving him two appendages, and that was really cool. We have some great animation tests that we did where he has to walk and open a door and drink out of a glass with just the two legs. A lot of times, you find that restrictions are your friend because they make something very specific or unique. But we found that…a lot of expressions count on the asymmetry of facial expressions. And Mike only has one eye! So we had a big bunch of figuring that out, and in the end we felt like, ‘Okay, we’ve got our hands full with the one-eye thing, so let’s give him two arms and two legs so that he can do things like carry a suitcase and walk at the same time.’

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I like this idea that Randall was this dapper tuxedo fiend at one point.

We were thinking that he would be kind of a kiss-up to the president of the company, Waternoose. That he is always dressing nicely and much more put-together than the slobby Sullivan. He sees himself as the next in line. In fact, there was a whole early draft where he was in cahoots with Waternoose. A whole separate thing of him being layers down below working under Waternoose. That’s in the movie, but anyway, we were trying to find ways to push him away from your sort of typical bad guy and bring layers to him.

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Finally, let’s talk Boo. She seemed to go through a lot of different ages and talking abilities over the development years.

Yeah, she was initially older. We were looking at, say, a relationship like in Paper Moon, where you have an older kid who’s a little wise to the ways of the world and probably smarter in a way that Sulley was at that time. But then as we ran into the voice thing, we realized she’s actually really appealing and maybe more interesting for Sulley if she is totally helpless. So in the early version when she’s tough, she walks in and kind of runs the room and Sulley definitely has his hands full. But in terms of appeal, what if she’s more like a kitten? Where if you let her alone in the city, she would get eaten or blown up or who knows what in a city full of monsters? So Sulley has to care for this kid, and that’s really what pushed him in his arc from being all about the work to becoming a dad in a way.

Last question: Is there a strange place you’ve ever seen this film in the wild?

Oh yeah. I’ve seen some really great looking — horribly great looking piñatas. Let’s see… we were in Jakarta last year doing press for Inside Out and found a piece of garbage which was branded with what was clearly a knock-off version of the film. So it shows up all over the place, that’s for sure.

EW also asked Docter about any possibility of a Monsters, Inc. 3 — see what he had to say here.