Not much was said in the episode about The White Hotel, the book that Peter reads and discusses with Rita. Like many things in the movie, it’s brought up in a clunky way with Peter saying “I read The White Hotel today” and expecting Rita (and the audience) to know what it’s about as if it was a book that everyone was talking about and in the cultural zeitgeist at the time. The White Hotel came out in 1981, 7 years before the play and over a decade before the movie.
But despite its awkward introduction, I can see why the playwright chose to have his characters discuss it. It is the story of a woman who overcomes her neurotic fears of life through her therapy with Sigmund Freud only to be exterminated by the Nazis during the Second World War. The book posits questions like, given the world's cruelty, should she have refrained from seeking help, from pursuing a cure? What was the point of all her efforts to find health if she was only to die so soon after acquiring it?
In Prelude I think Rita is meant to be a woman consumed with fear. Her insomnia, her reluctance to have kids, her questions to Peter right before the wedding, her getting her veil caught on her sleeve, and her tripping in the aisle are all meant to reflect that anxiety. And so what the play/movie is trying to suggest is that we cannot, indeed should not, govern our present lives by our fears for the future. We do not know what life holds, but we do have the capacity to make choices in the present that can make us happy in the present.
The only problem is that Meg Ryan has such an effervescent presence on screen, that those fears come off more like quirks than deep-seated anxieties.