Part of its success could well be attributed to the amount of research that has gone into it. Docter says that he began his co-writing duties by talking to brain scientists and behavioural experts.

"This is something that John Lasseter has espoused from the beginning of our filmmaking careers: research is essential," he says of Pixar's chief creative officer, who's better known to the public as the director of the first two films.

"It changed the way we approached the film as we learned more about the human mind. How many emotions are there? How do memories work? Why do dreams exist? We had a lot to work with, and it shows in the film."

In his high-concept drama, a pre-teen girl named Riley (voiced by Kaitlyn Dias) has a hard time adapting after moving to San Francisco with her parents. Docter chose five of her emotions - Joy (Amy Poehler), Sadness (Phyllis Smith), Anger (Lewis Black), Fear (Bill Hader) and Disgust (Mindy Kaling) - as the film's protagonists after considering many more.

"A lot of scientists said there are four basic emotions, others said 16 and some up to about 27," says the filmmaker, who eventually based his premise on six universal emotions proposed by US psychologist Dr Paul Ekman.

Surprise was taken out because it "seemed very similar as a cartoon character to Fear. We ended up with five because that felt like a good ensemble comedy," says Docter, who cites and "any number of good TV shows that play off of ensemble comedy" as his inspiration.

Pixar buffs would agree that Docter is prone to putting his personal life into his work. was made after the birth of his first child, Nicholas, and 's lonely widower character took up a bit of Docter's own introverted nature. recalls both his Minnesota childhood and the moodswings of his then 11-year-old daughter, Elie, now 16.

But he's not the only person at Pixar doing that, the director says. "I think everybody puts themselves into their films. I mean, if you look at , for example, that is really a portrait of John [Lasseter] and his life and all it's asked of him.

"And I think the same could be true of watching [director] Andrew Stanton and his interaction with his son in . We all have this kind of personal connection to the story, and that's ultimately what makes it emotional and relatable to people."

This is quite an understatement. By putting together one of the most historically significant runs of form by any studio between 1995's and its third instalment in 2010, Pixar has turned into a cultural institution that enjoys cult-like status among its many followers.

Docter sees two major factors behind that. One is the "system" set up by Edwin Catmull (the computer scientist who co-founded Pixar) and Lasseter, and supported by all contributors in their own ways.

"By that system we're able to - or even expected to - make mistakes," says Docter. "Films in their infancies are really pretty bad, but we all trust the process."

The other essential factor is "the quality of the brains that we have", he adds. "We have folks like Andrew Stanton, who is wonderful at assessing structurally the shortcomings of films; and John Lasseter, who is much more intuitive. He understands at the gut level what audiences enjoy, what attracts them and what makes a film fun to watch."

Since he joined Pixar in 1989 as the studio's third animator, Docter, 46, has witnessed its growth "from a staff of 100 people to 1,200"; he considers change an essential tactic for survival. "If we tried to remain the same since we started, it would fail," he says.

But some would argue that's already happening: the studio's (2011), (2012) and (2013) all met with comparatively lukewarm receptions.

While has temporarily averted the fear that Pixar has creatively reached a plateau, it's a concern that most of its upcoming projects - , , and - are sequels.

"The more Pixar films that exist out there, the one thing that becomes more difficult is to continue to do new things, to make films that have not been seen before," Docter says. "That's definitely something we'll continue to strive to do."

In his opinion, the biggest challenge for the studio to leap forward involves its efforts to remain relevant and keep up-to-date with technology.

"Obviously, between mobile devices, YouTube and all these other things, people are starting to ingest media and talk to each other in different ways. We'd want to be involved in that in some way," says Docter.

"We're going to continue and try to make long-format feature films, but I think we need to be looking at some of these other areas as well."

For a creative mind who delved deep into neuroscience and psychological research just to make a cartoon, those exertions probably wouldn't seem like too much.

Pete Docter's favourite five animated movies

1. Dumbo (1941)

"On the top of the list is this simple yet very emotional film. When my son was growing up he would watch it every day for two years - and I never got sick of it."

2. Cinderella (1950)

"This Disney version allows for great comedy, but always with the plot in mind - things really take off in the last third. It has a great sense of pacing and energy."

3. My Neighbour Totoro (1988)

"It's like you can smell and taste and feel the things that the characters are going through - thanks to [Hayao] Miyazaki's amazing direction and attention to detail."

4. Song of the Sea (2014)

"Unlike most modern films that are very manic and fast-paced, this film by Tomm Moore takes its time and allows you to enjoy the truth in the situations."

5. The Wrong Trousers (1993)

"I like Nick Park's films quite a lot. This is a short film, not a feature; there's such a sense of charm and personality to its characters. It's very, very appealing."

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