Richard Harris only played Dumbledore because his granddaughter threatened never to speak to him again!

Richard Harris was an Irish actor, singer, director, and writer best known by film enthusiasts for his role as King Arthur in the 1967 film Camelot. But, in the mainstream, he is most notorious for playing Dumbledore in the first two Harry Potter films before his unfortunate death. The role was his last one.

Harris had stated that he did not intend to take the part at first because of his declining health. However, his 11-year old granddaughter threatened to never speak to him again if he didn’t take it, and so he finally caved. In an interview with the Toronto Star, Harris expressed concern that his association with the Harry Potter moves would outshine the rest of his career.

He said, "Because, you see, I don't just want to be remembered for being in those bloody films, and I'm afraid that's what's going to happen to me."

(Source)