Can I perform in the theatre on my B1/B2 Visa or ESTA if I am not paid but just receive a stipend for expenses?
If it is a professional production, this would be a violation of your visitor status. The prohibition on unpaid work protects the labor market from foreign workers being brought in and working without pay. It also protects such workers from being brought to the US and being forced to work without pay.
If the production is fully amateur, it's probably okay, but the prospect of being reimbursed for expenses suggests that this is probably not the case.