I remember listening to an interview with either Jim Dale or Stephen Fry - who are the actors who read the Harry Potter series - and the particular actor said that he had recorded the accents of each of the characters and reviewed them before each chapter and possibly before each segment. We have a recording studio in the back of our house where music is made and I'm amazed at what my son-in-law can do with Protools.
Television series have notebooks filled with information about characters not only for the writers but for the actors and directors to review them for enriching their understanding of the character. It's impossible to hold everything in memory over years and years.
Character Pages are the same thing as recording accents and speech patterns except that you keep the visuals as well as the auditory. On Character Pages you write down preferred swear words, eccentricities, identifying habits and patterns, special things like physical features, or preferred clothing choices.
In Typhoid Mary - which will be the story I am working on for National Novel Writing Month - one of the characters will be very much like a woman I knew in Vermont named Constance Perry. She used to punctuate phrases with "Don't you know?" which has a wonderful old fashioned cadence in my ear. Typhoid Mary is known to have had a foul mouth - well in 1906 foul mouths are much milder than in 2010 - so how foul am I going to let her get without stepping out of period? Since National Novel Writing Month begins tomorrow I'm going to spend today creating and reviewing character pages.
Character Pages are begun as you outline and maintained as you write. With the advent of the computer you can always go back through the book or movie and add in bits and pieces that expand on the knowledge of each character.
Is it necessary to do all this work around writing? Absolutely not. It depends on who you are as a writer and how good you want your book to be.