It was sad to hear of Michael Gambon’s death, but I did think he was badly miscast as Dumbledore. Dumbledore is - or at least acts as - a whimsical eccentric with astonishingly garish fashion sense and a puckish sense of humour. But Gambon generally played dark, inscrutable roles, and seemed to me uncomfortable in this one. His Dumbledore always made me think of Rowan Williams trying to shout over a particularly bad-tempered synod. I thought that Richard Harris was a lot closer to how he is portrayed in the books, although he was a bit too reverential.
Apparently Patrick McGoohan was one of the actors originally considered for the role, but turned it down because of ill-health. Now he would have been very interesting!
I guess it's an advantage to not know what some of these actors have been in previously. I had no preconceived ideas about most of them. The only ones I'd seen in other movies and shows were John Hurt (I, Claudius, Doctor Who, and numerous movies), Kenneth Branagh (Shakespeare movies), Emma Thompson (Shakespeare movies), Helena Bonham-Carter (Lady Jane), David Tennant (Doctor Who), and Alan Rickman (Robin Hood: Prince of Thieves).
It wasn't until recently that I realized that Imelda Staunton had also been in
Much Ado About Nothing. So I'd seen her previously as well, but had never bothered to find out who played her character.
Oh, and of course I'd seen Robbie Coltrane in something, but I'm still blanking on what it was.
Now, for some weird casting... Downton Abbey. Maggie Smith and Imelda Staunton in the first movie, playing adversaries. For a moment during one of their confrontations, I half-expected the wands to come out.
And Merlin. I'm half-tempted to write a fanfic in which Kilgharrah the Great Dragon (voiced by John Hurt) didn't really die, but rather transformed himself into Olivander the wandmaker.