The 12th Doctor, Peter Capaldi, was nearly the secretary of the Doctor Who fan club.

From IMDB: "I'm very aware of the rich history of Chris [Christopher Eccleston] and David [David Tennant], and I've gone back and watched a lot of those episodes. And the previous history? I have to be honest, I can't claim to have watched the show in its entirety or have been a follower as avidly as some people are - and that's what I'm realising, that people love this show, people are avid, avid fans of it, and know it, and know its history and are part of it. I can't claim to know it as well as that, but I'm going to make it my business to do so, and fall into it in as much depth as I possibly can."

The same could be said of 7 of 9 actress Jeri Ryan, IIRC.
She'd never seen an episode of Star Trek before being cast in a major role, that eventually made her the icon/mascot for the whole Voyager series.