I wanted to make a proper tutorial out of this one, but no time for that, hence just few wip shots in a gif file (a bit big, so wait for it to load):
What you can see here is my usual workflow: first a rushed, dirty sketch. Some flat colours to get a general colour scheme. Then detailing the faces too much, which I always do, even though I know it would be better to work on everything at once. But somehow if I don't get the faces right at first, I quickly lose the interest with the rest. And after that, it's just lots of regular brushwork. One thing you can see here that works wonders in pictures is: adding shadows. Even if the picture has a diffused light and no big contrasts, just small hints of shadows (see last steps) really add to the depth of the whole thing.