When Sam materialises out of the stream of gold light-smoke that settles down next to Kate, he looks like Will, right down to the mannerisms and clothes. It's not an attempt to trick Kate, he's just in one of those moods where he wants to be just a tiny bit mischevious.

Which is to say - A bad mood, and possibly one where he doesn't want to look like himself.

"Hey. You seem to spend a whole lot of time practicing with your bow." His voice has the rather odd effect of sounding like somebody overlaid his own voice over Will's.