I find it really hard to 'cast' characters, though I know a lot of people do it. I'm not that visual, I think--I have pictures in my head, but they almost tend to be more pictures of the words describing the person than the person themself. (That is a strange way to put it and probably not clear at all, but oh well, I do my best!)

I totally know what you mean about being able to read anything Sherlock now you have the characters in your head! I hadn't written any at all, but I had watched the Downey/Law film and the BBC series (and read Doyle of course), and so then when I got a chance to write steampunk Holmes erotica, that was all in my head and informing the characters I ended up with--even though I wasn't writing about them, exactly, but really enjoying making my own versions of them.

I do tend to have a 'type', though, I confess. Other than my Harry/Draco days I almost always go for the 'best friends are more than friends' tropes, where the characters balance each other out (and usually one is dark and one is fair...).