Legally, where do I stand when my story is finished and assuming it is published...?

I would think that if you use anyone's name who is still alive you would need their permission to avoid the risk of legal action. It depends, however, who it is and how it is used. If, for example you said something like : 'President Obama is to visit Denver.' then that would be OK as it is based on someone who may do such an action, is a public figure, and is not defamatory. However if you said that Jack Nicholson got drunk in a bar in Los Angeles, then that could be construed as defamatory, even if he had done so. Remember that people may be famous for different reasons. An actor or singer may be world famous because of their films or records but they are still private individuals. Obama could be seen as a public figure because that's what he is. Also famous people may not want their names being used in case it jeopardises future work. Imagine if they were about to be offered a job that was mirrored in your book. If they lost it they would sue you for megabucks. If they were criminals they may be even more inclined to sue on the grounds that they have reformed and your book may further damage their reputation even if based on fact as your's would be a work of fiction.