Have you ever seen a cover illustration and thought, 'Hang on, that character face looks familiar. It looks like ... like ... why, it looks just like Jeremy Irons!'

Well, if you did, you were right. This is something you learn from working in a publishing company with tight budgets. The way it happens is this: a cover artist has to paint a picture that fits the description the editor has given him. He won't have read the book; he'll just know what the editor says about it. This will go as follows:

Character description: John - dark hair, dark eyes, genteel type, serious-looking, smartly-dressed, scar on left cheek, tall.

Now, the artist will find it easier to paint a convincing face if he has something to work from, but if he's on a limited budget, he can't use a real person. Hiring models is expensive. So what does he do? He'll pick up a magazine. Sometimes he'll vary the face a bit, but otherwise, you get Brad Pitt stepping down from a Victorian carriage, Cameron Diaz watching the Roundheads go past her window, David Boreanaz emerging from the trenches... It can be a bit surprising, but once you get into it, there's something fun about it. After all, 'who would you cast to play so-and-so' is a common game that gets played when it comes to fiction; in this case, the cover artist gets their bid in first.