"I am a tomboy, but maybe not as far as Arya," Williams told PEOPLE at the opening of a Game of Thrones exhibit sponsored by HBO and Time Warner Cable in New York City.
"She likes to play in the mud and stuff, which I do like to do, but I quite like to get it clean as soon as possible."
Last viewers left off with Arya in the show's second season, the youngest Stark daughter had escaped from her imprisonment at Harrenhal with companions Gendry (Joe Dempsie) and Hot Pie (Ben Hawkey).
"She always gets up to mischief," Williams, 15, said. "Arya has a tendency to speak her mind, and I think I'm not like her in that way. I struggle to come up to people and say what I think."
Game of Thrones's third season premieres Sunday on HBO at 9 p.m.