How they look.

For some of my characters, it was important for me to be able to see them as I was writing them. That way I found it easier to know if they were doing it right.
Ferguson, for example, is Liam Neeson.

This is him from Rob Roy. I know that's a long time ago, but the image of a man with a fight on his hands and no technology that would save him fits. Once he was in place, it became easy to hear the voice and work out if I was getting that right too.
Kayley I've always thought of as Thandie Newton.

I think she's got the kind of steel that the Mage character has and she definitely matches what the character Jessica later says of her, that she was the only one in the books who was sexy. We're talking character here, as much as looks.

Malaika has always been a young Aishwarya Rai. That doesn't fit the character description too well, as a few people say that they'd taken Malaika for being picky/haughty. I don't think that is something that maps onto Ash very well.
I've never had a good image of Adam. The closest that I've ever been able to imagine for his Brendan character is the boy who plays Seamus Finnigan in Harry Potter.

His adult self is a bit nondescript, which makes him hard to visualize.
Jock started off as a young Billy Connolly,

but changed into Simon Pegg after I saw him in Star Trek as Scotty. This image

works really well for me.

Jake was written as Jake Thackray, a singer/songwriter from North Yorkshire.

 His songs are mentioned in the book. Adam has never heard any of them, which he really ought to have done. I have to come up with a good explanation of that yet. Jake's dead now, so it would have to be a young version of him in the story. The pic above gets over the intense side of his character that Adam talks about. This:

 

says a lot more about the humour of the man.
I do have images in my mind for Megs and for Feri, but they are people that I know from work in different places, so I don't want to post pics of them here.