Why can some roles be recast successfully, and others not?
IMHO a big part of the reason that "Solo" was a financial disappointment is that you just can't recast the role of Han Solo. Yet the bulk of the fandom seems to be perfectly happy with Ewan McGregor taking over the role of Obi-Wan Kenobi, and some (myself included) say they'd like to see more of him in the role!
I consider this to be a pretty important question, with #9 being in production and all, in which Princess Leia was originally supposed to have a large role. Could another actress take over the role, and if not, why not? Did McGregor succeed just because the original role was so small that it could easily be expanded on, or did Guinness just lack a certain magic that made him unique?
Because I think that was the case with Harrison Ford in the role of Solo, he's not a great actor but somehow he managed to be so perfect that he can't be replaced. Well, in the first two films, anyway.