A wizard is never miscast. He’s always played precisely by who he’s meant to. In the case of Gandalf, that person was Sir Ian McKellen. In the case of Dumbledore, there were two actors: Michael Gambon for the latter six films and the late, great Richard Harris for the first two. But now it turns out that after Harris’ passing, the Harry Potter crew approached McKellen about taking the role before they offered it to Gambon.
McKellen — the spitting of image of Gambon — would have been a good fit, but he says he turned it down because Richard Harris once criticized his acting. As he told the BBC:
When they called me up and said would I be interested in being in the Harry Potter films, they wouldn’t say what part but I worked out what they were thinking. I couldn’t take over the part from an actor who I know disapproved of me.
Evidently, Harris had once called McKellen’s acting “technically brilliant, but passionless.”
So Harris must have never seen The Fellowship of the Ring, but it all worked out for the best. McKellen says he’s an avowed fan of Gambon’s performance, who he says played Dumbledore “gloriously” and anyway, that would have made Sir Ian both Gandalf and Magneto and Dumbledore and Sherlock Holmes AND Cogsworth, and that’s just a few too many iconic roles for one man.