I don't do this. I am amazed/amused that other people do... personally, I think appearance is the least important factor in my thinking of character, and while voice/mannerisms is more important, I find that changes so much with the role for an actor that it's unreliable for me. If I use models, I pick people I've met.

Further, and this is the iffy thing, I don't like to write someone if I can't imagine how they smell, and I can't imagine how actors smell.