Who is Denham Fouts, Barbara Hutton, Montgomery Clift, Tallulah Ban from Answered Prayers: The Unfinished Novel and what is their importance?

Throughout the book, the narrator describes encounters with several "real life" individuals that include the gigolo and writer's muse Fouts, socialite Hutton, movie star Clift, theater stars Bankhead and Winwood, columnist Parker, former First Lady and fashion icon Kennedy.