I find that it's their names that influence my idea of what the characters look like; I get a picture of them initially that's more beautiful/handsome/ugly than people I've know of the same name but generally similar to them.
I may change my perception of them a bit as I learn more about the characters but they definitely always look like people I've known.

Names do it for me too, but also how other characters treat them and how they deal with it.

The mental image comes from a combination of the descriptions of the charcaters given by the author combined to quite a large extent by the setting of the action. For example, in Mervyn Peake's Gormenghast trilogy, the main protagonists are all well described but the superb descriptions of the castle and the surroundings all conveyed to me somewhat medieval times. You can imagine my surprise when all that needed to be re-visualised when, I think it was in the third book, a car appears on the scene!!

I don't think that I draw any of my images from people I know.