Have watched and trying not to read this thread because I haven't read the book. What the first episode clearly shows that someone can clearly loose the plot when it comes to the thrill of an affair, and Emily Watson plays this well, including the glow and the smiles (you're looking well) and trying to keep it together. She is intelligent, and is carried away. It happens.
She is also married to a man who is deemed attractive, something she no longer sees, and she is saying that he may be having an affair in order to assuage her own guilty feelings. This is her story, those around her are mere distractions, including her husband and his life. Her affair partner might not be standard good looking, but obviously she found something that was akin to animal passion. Even intelligent people fall for this.
The ending was shocking. From a viewers point of view I could see perhaps a clumsy attempt at a fumble when she went to his office, but I certainly wasn't expecting such a violent scene. Can't wait to see how this plays out.