In the TV Series we see Arya wear various faces, initially the faces seem to be similar to her, young females that she could easily take on the mannerisms and possibly learn to put a voice on for. The faces she wears here we have never seen before and so can't compare the voice or mannerisms to the original wearer.
However when she dons the face of Walder Frey she perfectly mimics his voice, his mannerisms, her body size and shape seem to match his own and she becomes almost a perfect copy. Considering she is a young small women at this point it seems that the face must do more then just allow her to look like the person she is imitating.
Has it ever been explained as to the powers or limits of the faces, are these powers shown in the books (a wearer looking and sounding like the original person), does the face change the person, or does it simply change the perception of all those looking at the wearer so they see and hear what they expect to?