How Do I ert Myself to this Person?

I am an unpaid volunteer stage managing a local play. I have gone to great trouble and some expense for this play. Before I got involved, I was told by the director it would include ONLY certain duties. Well, the istant director has turned out to be this woman I CANNOT stand from a cl I took. She is bossy, self-deluded, and pompous. She does not perform istant director duties, but only seems to set out snacks... when she's not bossing me around and talking down to me. I feel abused by this woman and like she has no business giving me any orders. I'm an unpaid volunteer who reports to the director, not her. Well, now she's trying to make me take out the huge loads of trash at the theater. How do I tell her no without being too mean or getting into a fight?