I thought about Ivy.The narrator continues on like this the whole while, declaring himself a loner while constantly clinging to the people around him, distracted and claiming his distractions are his real life, pointing out how rational and sensible he is, how grounded in reason and science his worldview is, all the while acting more and more irrationally. His employer has suggested that the narrator needs a vacation. The narrator wonders why his stomach hurts constantly, then assures the reader that he has never been in better health. I assume that "I forgot to phone up Williams, even though I had been thinking about it the whole time" means that the narrator was thinking about phoning up Williams the whole time he and Ivy were having sex.
When I held Ivy I was also thinking: I should have my film developed, phone up Williams! I could solve chess problems in my head, while Ivy said I'm happy, o Dear, so happy, o Dear, o Dear! I felt her ten fingers on the back of my head, saw her epileptically happy mouth and the picture on the wall--it was hanging crooked again--I heard the elevator, I tried to figure out what the day's date was, I heard her question--You're happy?--and I shut my eyes, in order to think about Ivy, who I held in my arms, and kissed by accident my own elbow. After that it was all forgotten. I forgot to phone up Williams, even though I had been thinking about it the whole time. I stood at the open window and finally smoked my cigarette while Ivy left the bedroom to make tea, and it suddenly occurred to me what the date was. But it played no role in my life that day, the actual date. Everything happened anyway! Then I heard that someone had come into the room and I turned to find Ivy in her dressing gown, bringing our two cups in, then I went to her and said: Ivy! and kissed her, she's such a good fellow, even if she doesn't get it that I prefer to be alone--
Frisch, in his Paris Review "Art of Fiction" interview, says that the narrator's prose is deliberately flat, self-consciously overlooking details, colors, beauty, the messages of the senses. The narrator, Walter Faber, says nothing because he denies everything even as his denial crumbles around his ears. "He has the arrogance to say nothing," Frisch says of Faber. Frisch goes on to explain why so many of his protagonists lead similar lives, and have such similar personal habits. The short answer: laziness on the part of the author. I like Frisch's honesty. "I grab the things that are here in the living room; I'm too lazy to go look at what's in the kitchen."