I'm not going to help my friend move
Why wouldn't you help them move?
I have theatre practice, I'm going to be busy.
If your friend really needs you, then I think you should cancel your other plans.  When friends really need help, their friends should be there for them.  Play practice can wait!
I am just extremely busy with this practice and I can't help him today.
It sounds like you're just making excuses. Why not just help your friend out? I'm sure it'd make them very happy.