Beware: Extracting information from a subject for a memoir can cross the fine line into a question and answer session that doesn't reveal any character. Phrasing questions to elicit paragraphs not single word or simple sentences without background is the key.
Think about college exams. There are pages of multiple choice and then the grand finale: the essay question. The instructions say, "Answer in complete sentences" but people don't always read (or heed) the directions. Students often start out with a dependent clause such as, "Because the event led to..."
The key to a successful memoir is to start with the essay question and then ask the test questions to clarify the answers. Sounds like a piece of cake. You sit down with your subject fully prepared and the session begins. Try to think like your subject and anticipate the direction his answers will take.
When you can't specify the length of the answer your subject may start rambling. This puts the ghost writer/editor in a precarious position. If you stop him, you may miss some key memory that contains the story that will make your book a success. If you let him wander down the stream of consciousness the result will most likely cause the editor a great deal of work later on.
Assume you are working with an elderly person who has many years filled with experiences. If not, you you'd be doing an interview not writing a memoir. An event that happened last year leads back to a colorful memory that happened in childhood that in some way relates, at least in the mind of the speaker, to what he was saying. The task of putting things in order is very difficult in the end process because your reader hasn't made the connection clear enough for the reader to follow.
The challenge is to let the subject talk, interrupt him to ask "Do you remember what year that was?" or "What city were you in when that happened?" in order to get some reference points that will help you later on.
Without these bits of information you may find yourself asking that age old question: Which came first -- The chicken or the egg?
Here is a link to an interview that could have gone terribly wrong if the subject was not young and expressive. Imagine saying to a 78 year old man, "Please introduce yourself."