[Sources can be found at the bottom of the article.]
1/15. The opening credits sequence was filmed at a real chocolate factory in Switzerland.
2/15. After reading the script, Gene Wilder said he would take the role of Willy Wonka under one condition: that he would be allowed to limp, then suddenly somersault in the scene when he first meets the children. When the director asked why, Wilder replied that having Wonka do this meant that "from that time on, no one will know if I'm lying or telling the truth." The director asked, "If I say no, you won't do the picture?", and Wilder said "I'm afraid that's the truth.”
3/15. When Willy Wonka (Gene Wilder) drinks from a flower-shaped cup and then eats the cup, the cup itself was made of wax. Wilder had to chew the wax pieces until the end of the take, at which point he spat them out.
4/15. The Wonkatania was on a track in the chocolate river, but the actor playing the Oompa Loompa at the helm thought he was actually steering it. For the sake of believability, director Mel Stuart didn't tell him the truth.