None of the answers quite fit, but I answered the last one, meaning, I don't particularly change the way I speak to my therapist from the way I speak to anyone else: I use the tone, inflection, and voice register appropriate to the situation. There are, perhaps, subtle adjustments, but not markedly so. I didn't say "in a professional way" because that felt too formal, stilted, and CYA to me. I'm probably least "natural" when dealing with someone professionally like an administrator, lawyer, accountant, etc. Friend is not right at all; the therapist is not my friend. Certainly not parent, etc.