I thought an actress in Dexter: New Blood looked like SMG, at least at some angles. But she didn't appear in the credits so it obviously wasn't her. But the resemblance was uncanny. And the voice as well. By the last episode I snatched a glimpse of "Guest starring: Sarah Michelle Gellar". I felt pretty stupid.
But what makes a role a guest role? She was in basically every episode. A normal supporting character. If there are more seasons she'll probably be playing the same role in every episode. Maybe I should pose that question to !television@piefed.social