When an Ancient takes the appearance of a human, they are not solid, as Athar was. In "Ascension" Orlin had no physical form until he became entirely human and lost his powers. In "Abyss" Daniel appeared to Jack as a human, but Jack was able to throw a shoe through him, so he obviously was not fully human.Originally Posted by AsgardCarnage
Athar, though, seemed to be able to alternate between forms at will. I believe the other Ancients gave her this unique ability so she could better defend Proculous. It is clear from everything we know about Ancients, that, with the obvious exception of Athar, once they retake human form, they cannot re-ascend without the others.