Greg Cox wrote:
If the original comic-book character is usually drawn with red hair and a mustache, fans immediately zero in on the most famous actor with red hair and a mustache, to the exclusion of other, equally suitable actors. Ditto for height, nationality, etc.
Although sometimes it works out perfectly. Probably everyone who was a fan of both ST:TNG and the X-Men pegged Patrick Stewart as the ideal Professor X ages before it actually happened. A college friend and I had that conversation sometime around 1989, I think.
(Although Xavier's look was actually modeled on Yul Brynner.)