When writing about real people, there are issues of copyright, libel, rights of privacy and rights of publicity to consider.
There are several permissions that a writer may need (or choose) to obtain to safeguard against such legal matters. Interview release forms, for example, can be signed by the interviewee to ensure the author free use of materials gathered during an interview session. Furthermore, some writers acquire “life story” rights (sometimes even when not legally required), in order to remove any threat of a lawsuit that could encumber a play. Writers may additionally wish to obtain permission from an individual or estate to use a range of copyrightable material (i.e. letters and diaries) within a work or to otherwise have access to materials and private information. Involving the subject in this way has the added benefit of bringing an air of legitimacy to a work. However, if the real person is controversial or particularly litigious, requesting such permission may unnecessarily put you on the subject’s radar.