There are some things the book does better and some things that movie does better. Spoilers ahead!!
So the book definitely went into details that explained their breakup better than the movie did. In the movie, Hubbell's behavior makes him look like deadbeat dad for the sake of being a deadbeat father, and certain things that Katie and Hubbell say don't make complete sense, such as being a bad loser without the political context of the breakup scene. However, when you add that Katie decided to divorce him because she was informed as a Communist sympathizer by a classmate, and they divorce so he can have a career in Hollywood, it makes more sense. It gives Katie the power because she makes the decision and shows strength, while Hubbell is heartbroken. Is he a deadbeat dad who ultimately chose his career over his family? Yes. But that makes him look better than losing contact with his daughter because he had an affair and divorced his wife. No phone calls? No letters? So you don't want to see your daughter because it is AWKWARD? Or you don't see your daughter because you could lose your career for keeping in touch with an ex-wife who is a supposed Communist sympathizer.
In addition, the book also discusses how the politics affected others in Hollywood with more depth. Hubbell and Katie have friends and colleagues who lose their careers over the blacklisting. Hubbell is allowed to be more flawed in the book. There's a line in the book that I wish that they kept in the movie. Finally she says, "I want us to love each other." To which he replies, "The trouble is, we do." There were certainly more changes in how Bissinger is a more likeable and complex character in the book than the movie.
However, like the other reviews stated, this is a case where the movie is better than the book. Robert Redford brings some depth to Hubbell's character that made him more compelling in the movie than the book. The script is tighter, and it works better as a movie. I enjoyed learning about the characters' back story and their parents, but I was more invested in Hubbell and Katie as a couple in the movie than the book. I wish the movie kept the breakup scene as shown in the book, which is the one thing that the book does better.
And that they definitely explained that initial drunk sex scene better in the book. Hubbell realized what he was doing, even if he was exhausted. And I wish they made it more clear in the movie.
The Way We Were is a movie that I love, but it would have been much better with a couple more scenes. The book was a nice supplement.