Hiring an Attorney
I can’t recall being asked about whether and how to maintain confidentiality of business plans and other information when interviewing possible attorneys whom one might want to retain until very recently, but I have been able to tell from my interactions with potential clients that may of them worry about what is OK to tell the attorney they are interviewing (and might or might not hire).
Attorneys, as a general matter, have a professional responsibility to keep such information confidential, and reputable attorneys will abide by these rules. Having said that, not all attorneys always meet this standard, so don't take a risk that you don't need to.
I don't think you need to get very deep into details of your business plan at an initial meeting with an attorney, and, in general, I would advise you not to. Also, don't leave a copy of the plan or your slide deck or other materials that contain sensitive information with any attorney, until you have selected one to work with and he or she has agreed to represent you.
A general high level description of your business should suffice for the initial meeting. Once you have entered into a formal attorney/client relationship, the matter is different. Once you are in a formal relationship, you should be able to rely on your attorney's duty to keep client information confidential. Attorney's typically do not sign NDAs, and I would not ask for one.