They have said that it was an extra that used the bathroom and accidentally came down during the take and saw they were filming and then went back up. Instead of cutting it out, they left it in, thinking it was funny the actor had a reaction that made it seem like he was avoiding the awkward scene with the "remembrance" of Livia and not because he didn't want to blow the take coming down the stairs. Just a "happy accident" that worked out. I love that guy's "oh shit!" reaction. I thought it was Uncle June at first. *it's not*