(NEXSTAR) – This week marks the 50th anniversary of “Willy Wonka & the Chocolate Factory,” the 1971 film adaptation of Roald Dahl’s whimsically absurd children’s book about an eccentric, reclusive candy manufacturer and the kids he invites on a tour of his factory.

The role of Willy Wonka, brought to life by Gene Wilder, still stands as one of the most enduring performances of Wilder’s career. But he only agreed to the role on one condition — leading to one of the movie’s more iconic scenes.

In an interview with film historian Robert Osborne in 2013, Wilder recalled his early meetings with “Willy Wonka” director Mel Stuart, as well as his initial reaction to the script.

“It’s very good. But there’s something missing,” Wilder remembered telling Stuart after reading the screenplay. According to Wilder, he then went on to demand the now-classic scene in which Wonka first greets his guests. In it, Wonka walks out of his factory with a cane, slowly and unsteadily, before falling and performing a somersault, revealing he’s not as feeble as he first appeared.

“If I play that part, I want to come out [of the factory] with a cane, [as if] something’s wrong with my leg,” Wilder told Osborne. “And come down the stairs slowly, and then have the cane stick into one of the bricks down there. And then get up, start to fall over, then roll around, and then they all laugh and they applaud.”

Stuart, perplexed, asked Wilder why he wanted to include the bizarre scene.

“I said, ‘Because from that time on, no one will know if I’m lying or telling the truth,’” Wilder remembered.

After filming, Stuart called Wilder and informed him that the producers loved the finished scene. And it’s a good thing they did, because otherwise it would have been replaced with another take — filmed against Wilder’s wishes — where Wonka didn’t fall.

“I should never have said, ‘I’ll do it one time without the [fall],’” Wilder said.

Luckily, Wilder’s idea made it into the finished film, along with many more of his artistic choices. One of them — Wonka’s angry reaction to learning that Charlie and his grandfather stole sips of his Fizzy Lifting Drinks — even surprised the director.

“In the last scene, Gene surprised me,” Stuart said in a Warner Bros. featurette released in honor of the film’s 50th anniversary. Specifically, Stuart was shocked to see “the violence of Gene’s temper” when Willy Wonka “explodes” on Charlie and his Grandpa Joe.  

“I still am always fascinated by Gene’s delivery,” added Stuart. “There’s nobody alive today that can do what he did in that picture. It’s a performance that ranks … in the hall of fame.”