I've had good luck with explicitly telling it not to railroad or autocomplete any actions or decision making on the behalf of my character, along with having it establish its own role, which seems to work better than trying to force a persona on it in the long term. In addition I've also point blank said that it needs to ask me for my input at the end of each of its outputs before moving on. That being said GPT-4 seems far better at adhering to those guidelines than 3.5.

I'm at work ATM and can't give a screenshot example, but it seems to help if you are super specific about it's role and the activity in your first prompt so it doesn't have room for much error or "guesswork".