Right now, there's a lot of buzz about who the next Doctor Who companion will be. I've heard some interesting names being floated for the part, but I've also seen some "here's who they should cast" lists. Those tend to be very frustrating for me, because most of the "dream cast" lists that you see on the Internet tend to boil down to, "Here are people who are already doing a ton of stuff in sci-fi/fantasy and who are up for every third part in any genre movie or TV series you'd care to name! Wouldn't it be awesome if they were in this, too?"
And while the answer is usually, "Yeah, sure," (I mean, who doesn't want to see David Tennant in more things?) it feels very insular and limiting. The honest answer, especially for Doctor Who, is that I want to see someone I've never heard of. I want to see someone who isn't on my dream casting list, or even my casting list at all. I don't have the resources that a casting director has, and I don't get tons of headshots and resumes crossing my desk. Why should I be lobbying for anyone to get a part?
No, I'd greatly prefer to discover a new actor or actress through Doctor Who, or through any number of other sci-fi/fantasy movies. Let them be the next Matt Smith, or the next Tatiana Maslany, or the next Michael B. Jordan. The casting director's job should be to range far and wide and discover someone brilliant--if they've only found the people we all know about already, they're not working hard enough.