Disowned Adaptation: By most accounts Roald Dahl hated the finished film, to the point where he refused to allow a movie of its sequel to be made and wouldn't release movie rights to his other children's novels for more than 15 years. This was due to the casting of Gene Wilder (Dahl insisted that Spike Milligan should have been cast in the role) and to the extensive rewrites done to his screenplay received without his foreknowledge.
Mrs. Bucket's song "Cheer Up, Charlie" is often cut from commercial TV airings, and this was the director's idea — he felt the song was superfluous by that point in the story. Since the late 1990s, ABC Family airings often cut the notoriously scary boat ride sequence.
ABC Family uses two different cuts of the film: a condensed one that runs for two hours, and a wholesale one that runs for two and a half. Which one they use depends on what else they want to air and, by extension, The mood that they're in.
The cast wasn't allowed to see the Chocolate Room set until the scene where they first emerge into the room was shot, so their reactions are genuine.
Charlie's reaction to Wonka declaring he would get nothing due to defying the contract ("Good day sir!") is also genuine; in rehearsals Peter Ostrum (Charlie) was not told that Gene Wilder (Wonka) would be shouting at him. Since Wilder and Ostrum had become friends on the set, Wilder desperately wanted to tell Ostrum that he would be shouting so that Ostrum wouldn't think that they had stopped being friends.
For the riverboat scene, Wonka's ranting poem was not in the script (it's a lift from the novel), hence the disturbed looks on the actors' faces, who thought Wilder was actually losing his mind.
Speaking of the riverboat, it was on a track under the water, but the Oompa-Loompa supposedly steering the boat wasn't told this.
During the scene with Wonka's somersault, Denise Nickerson (Violet) genuinely thought that Gene Wilder (Wonka) had injured himself.
Mel Stuart deliberately ran Paris Themmen through the lines when Mike explained the technicalities of television to Wonka in the Wonkavision Room so often that Stuart got the right "nasty and pissy" attitude from Themmen he wanted for the scene, as that was how Themmen naturally felt at that point.
Fake Brit: Paris Themmen mentions in the DVD Commentary that he and Denise Nickerson "hung around Julie (Dawn Cole, who played Veruca) too much" and Julie's British accent rubbed off on them as a result. Paris points out certain lines in the film where he and Denise affected British accents for no good reason, and the takes were kept. He also discussed that loved ones watching the film could not recognize that the lines were spoken by Violet or Mike and not by Veruca.
Julie Dawn Cole, who played Veruca, swiped a few props from the set, including a Golden Ticket and an Everlasting Gobstopper. However, she lost the mink coat that was made specifically for the movie, having apparently left it on the back of her chair when she went to lunch. The director yelled at her until she cried when he found out- it was real fur.
Not only that, but she also got a peek at the Chocolate Room early (the other children only saw it during the main filming).
On a darker note, Denise Nickerson (who played Violet), made her scenes and returned to school... and then, after two days, her face and hands start to turn blue! Oh Crap!! It turned out that the paint used to make her face and hands blue went deep under her skin, and then slowly resurfaced during next few weeks. Talk about art imitating life!
The actors actually behave a lot like their respective characters during the DVD Commentary:
Denise Nickerson, Julie Dawn Cole, and Paris Themmen have a lively conversation throughout, with Themmen periodically making witty observations along the way;
Peter Ostrum was basically Mr. Exposition, having been the first of the five main children to arrive for filming and the last to leave;
Michael Bollner says very little, and then only when prompted;
In a somewhat related note, 2011 saw the release of I Want It Now!, Julie Dawn Cole's memoirs, featuring, in-depth, a first-hand recollection of her experiences filming Willy Wonka, as well as the subsequent reunions and commemorations of the movie over the years, with plenty of rare photos taken during the movie's production. As of 2015, she is the first and only "Wonka kid" to have published any recollections of making the film.
Romance on the Set: Julie Dawn Cole and Denise Nickerson both had a crush on Peter Ostrum, so they would hang out with him on alternating days. It was a congenial version of the Type 3Love Triangle.
Studio Hop: Paramount distributed the movie during its premiere. After it flopped, they decided not to renew distribution rights. Warner Bros. then added the movie to their library, where it belongs to this day, and went on to produce two further adaptations of the original source material (the 2005 film and 2013 West End musical).
Vindicated by Cable: The film was a failure at the box office, but has become a classic in the years since.
What Could Have Been: Jean Stapleton was offerred the role of Mike Teevee's mother at the same time she was also being offered the role of Edith Bunker. Even the director, who begged her to be in the movie, later admitted that she made the right choice.
Lyricist Leslie Bricusse claims that he pressured the producers to hire Fred Astaire for the part of Wonka, but that they didn't listen to him.
Joel Grey was also considered for the Wonka role. While he was a good match for the character's description in the novel, the filmmakers didn't think he would have the larger-than-life, commanding presence the character requires on screen, since he wasn't much taller than the kids would be.
The songwriters wanted Sammy Davis Jr. to play Bill the candy shop owner, but director Mel Stuart felt that Stunt Casting the role would be too distracting for the audience and hurt the distinction between the real world the first half of the film takes place in and the unreal world of the factory. He nixed the idea of having "I've Got a Golden Ticket" become a Crowd Song for similar reasons.
Roald Dahl reportedly wanted Spike Milligan to play the part of Willy Wonka. The fact that Milligan was not cast as Wonka was allegedly one of the reasons Dahl refused to allow Charlie And The Great Glass Elevator to be made into a movie, even writing that decision into his will. (Dahl also would have been happy with Peter Sellers, but Milligan was his first choice.)