Fear makes companions of us all.

Doctor Who has dealt with the concept of fear a few times in the past ("Kinda" (1982) and "Hide" (2013) come to mind), but never with the personal scope that "Listen" employed. Having a monster that may or may not exist really let the episode feature some real character moments for the three principles. The Doctor, Clara, and Danny all get their moments in the spotlight here. If the Doctor got his share  of fear as a driving factor for his character, then Clara acted as the counter balance of that fear. Season 8 continues to develop her character in a way that was sorely lacking last season. Character arcs aren't the only thing being expanded here. This episode deepens the mythology of the show by making it an integral part of the thematic elements at work. It will be interesting to see how this is played with over the course of the next fifty years.