ANAHEIM, California -- It's been nearly 20 years since "Toy Story" hit theaters. As the first feature-length computer animated film ever, "Toy Story" set a precedent for innovation and catapulted Pixar Animation Studios into the forefront of animation.

But it wasn't easy to pioneer an entirely new medium. In the early '90s, Pixar was mostly known for its short films and television commercials. With Disney, they were able to take the chance of a lifetime on one very simple, yet entertaining idea: what if your toys came to life when you weren't looking?

To celebrate "Toy Story" turning the big 2-0 this November, Disney animation head John Lasseter (who directed "Toy Story" and its sequels) reunited some of the film's VIPs, including key members of the Pixar brain trust Pete Docter, Andrew Stanton and Lee Unkrich, for a special 20th anniversary panel at D23 on Saturday (August 15). Here's what we learned during Lasseter and co.'s walk down memory lane:

"Nightmare Before Christmas" paved the way for "Toy Story."

We can all thank Jack Skellington (OK, and Tim Burton) for Disney's willingness to give "Toy Story" a shot. Prior to "Nightmare Before Christmas," no animated film bearing the Disney name was ever made outside of Disney. But Burton's macabre stop-motion feature opened the studio's minds to the possibilities of collaborating. "Because of 'Nightmare Before Christmas,' 'Toy Story' happened," Lasseter said. (Fun fact: Lasseter and Burton were classmates at CalArts.)

Pixar didn't want to make another Disney film.

"We came up with a list of what we wanted our film not to be," Lasseter said. "We didn't want it to be a fairytale. We didn't want a film where the bad guy grows bigger in the third act."

Lasseter and co. were also tired of boring protagonists, citing "Aladdin" as a specific example of a great movie with a protagonist that is almost too perfect. "Why do they name these movies after the most boring characters in the film?" snarked Stanton.

Pixar, however, wanted protagonists with flaws because that was the only way to make them interesting. "Having our main characters be the most entertaining characters in the film" was a top priority for the team.

Buzz and Woody looked A LOT different in the beginning.

Buzz Lightyear was originally named Tempest, after the animators' obsession with the Atari game Tempest, and he was only six inches high. Other possible names? "Tempest from Morph to Star Command," Lasseter recalled. One thing that was always part of the mix, though?

"To infinity and beyond" -- an iconic line that was part of the short pitch video that they made for Disney, which was screened during the panel.

Meanwhile, Woody was a large ventriloquist dummy -- who was kinda unsettling to look at. Several notes criticized Woody for being too creepy, so Lasseter eventually decided to make Woody a pull-string toy (inspired for his own love of one as a child) instead of a dummy. In the end, Buzz was scaled to 12 inches and Woody to 15 inches.

As for Buzz's iconic spacesuit, in earlier storyboards -- back when he was a wee, 6-inch Buzz -- his suit was red. After deciding to model it after IRL spacesuits, the team decided on white with lime green and purple accents. Why those colors? "Lime green is my favorite color and purple is my wife Nancy's favorite color," said Lasseter. "Just like she and I, they go really well together."

Fun fact: Lasseter's wife is also the inspiration for Bo Peep.

The first sequence they put into animation was the toy soldier scene.

In order to better understand how the toy soldiers move, Docter decided to nail his own sneakers to a wooden board. The animator unfortunately nailed them from the bottom on his first attempt ("That's so you," said Stanton.) Docter later made these prototypes for the entire team and they spent an entire day moving around with their shoes nailed to wooden planks. Docter also sewed together his own Woody doll during the production.

Some of Woody's most memorable bits of dialogue were improvised by Tom Hanks.

"In the process of making an animated film, we always record the dialogue before we do the animation so the animators can be inspired by the actor's movements," said Lasseter. And boy, were they inspired by Hanks. So inspired, in fact, that they're still using Hanks' outtakes in "Toy Story" films because he "gave them so much."

According to Unkrich, who served as editor, bits of Woody's dialogue in "Toy Story 2," "Toy Story 3" and the upcoming "Toy Story 4" were recorded by Hanks in 1994.

Hanks was also a master improver with props. "We found out that if we gave him props, he would just come up with ideas," Lasseter said. One prop -- a fake severed arm that Lasseter borrowed from his kids -- led to the sequence where Woody plays with Buzz's detached arm in the window, like a shadow puppet.

"All of that dialogue was Tom Hanks ad libbing," Lasseter said.

Joss Whedon wrote the most iconic line from the movie.

After reading a early draft of Whedon's "Buffy the Vampire Slayer" movie that never was, Lasseter was so impressed that he brought the young writer into the writers' room to help with the script.

Whedon eventually came up with one of Lasseter's favorite lines in any Pixar movie: "You're a sad, strange little man, and you have my pity."

They changed Buzz's character to better match Tim Allen's performance.

After Allen had recorded his dialogue, it became very clear that the it didn't quite match who Buzz was, initially. This was especially hard for Docter, who voiced Buzz for all of the early treatments.

"Originally, we thought Buzz was aware of the TV show from which he can from -- it was very meta," he said. But once they listened to Allen's take, they rewrote Buzz's dialogue and instead, made it that Buzz doesn't know he's a toy from a TV show.

Disney wanted it to be a musical.

"Toy Story" was being made during the era of Disney's musical renaissance. Animated features like "The Little Mermaid," "Beauty and the Beast" and "Aladdin" were huge successes for the studio. So when Disney met with Lasseter and co. well into production on "Toy Story," they brought along a few (read: "about six or seven") songs for the film.

When Lasseter broke the news to the poor guy that "Toy Story" wasn't going to be a musical, "his face went completely white. I think he thought he was going to be fired."

Of course, "Toy Story" did need a song, and Lasseter and his team knew from the start that there's was only one guy suitable for the job: Randy Newman.

"There was so much sincerity in his insincere delivery -- and that was what 'Toy Story' was all about," said Stanton.