If the petition is based on i-130, then my experience says the lawyer can help you prepare papers and all and even accompany you to the interview but they are not allowed to speak at all, unless they are asked by the USCIS interviewer. You have to speak up yourself. But we do hire lawyers for the peace of mind, and they make hundreds of dollars per hour yes. Lawyers will talk to you in technical language and it can be assuring for some people. My experience says most of the times, they themselves are not sure what the USCIS officers are going to do or say.

P.S. are you in touch with the lawyer who helped you file the i-130 in the first place?