Expression impression

I was thinking about school days recently and I recalled a friend and I each being able to do a recognisable impression of someone in the year below simply by pulling a certain facial expression. Absolutely no words or bodily actions were required.

I’m sure he cannot have walked around 24/7 with exactly that expression on his face, for that would not allow him to exhibit the whole range of emotions that humans are capable of displaying.

So what was special about that particular expression? Or perhaps it was that his facial structure lent itself to falling naturally into that sort of position? Does anyone else know of anyone who can be successfully mimicked using just one facial position? If so, what do you think is the reason behind the success of the impression?