In the Season 2 episode "Halloween", Willow is turned into a ghost and is thus completely insubstantial and goes walking through walls and the like. However, when she and Giles are in the back room of Ethan's shop and Giles tells her to leave, she brushes a curtain hanging in the doorway with her shoulder on the way and the curtain moves, continuing to sway as she walks off, it's very obvious.