In "Morning," Patrick gives a long list of his apartment's garish, expensive decor. He describes his coffee table laden with fancy ashtrays—but, ironically, Patrick clarifies that he does not use them:
A glass-top coffee table with oak legs by Turchin sits in front of the sofa, with Steuben glass animals placed strategically around expensive crystal ashtrays from Fortunoff, though I don’t smoke.
Patrick's disuse of his expensive possessions is one of the fundamental ironies of the novel: he makes personal choices exclusively for aesthetics, even when they explicitly (by his own admission) do not align with his actual life. He buys the ashtrays so that his houseguests can notice his taste and wealth, even if they are apparently otherwise useless to him. This irony is greater given the fact that Patrick assaults and murders most of his houseguests in the novel.
But there is another layer to this irony, because Patrick indeed does smoke. Patrick smokes cigarettes while drunk or high and has smoked cigars since he was a student at Harvard, as he discusses with his old girlfriend Bethany. This is an example of Patrick as an unreliable narrator—here he flatly lies to the reader, seemingly to protect his healthy image. Patrick's life is full of ironies and lies, both toward the people around him and the reader.
In the chapter "Deck Chairs," Patrick has dinner at the titular "California classic" restaurant with Courtney, as well as Anne and Scott Smiley. Anne wants a rum and Diet Coke, but Patrick recommends forcefully that she choose Diet Pepsi instead. His hyperbolic reaction is shocking:
“You should have the Diet Pepsi instead of the Diet Coke,” I say. “It’s much better. It’s fizzier. It has a cleaner taste. It mixes better with rum and has a lower sodium content.”
The waiter, Scott, Anne, and even Courtney—they all stare at me as if I’ve offered some kind of diabolical, apocalyptic observation, as if I were shattering a myth highly held, or destroying an oath that was solemnly regarded [...]
Patrick interprets the reaction of everyone present as extreme shock. This seems patently ridiculous since Anne and Scott are always especially genial and, in any case, this is such a benign statement that no one could react to it in the way Patrick imagines. But, because of Patrick's first-person narration, it is unclear what the rest of the characters actually think. Patrick's delusions and intense social difficulties make him an unreliable narrator, clouding the reader's view of what actually goes on around him.
Patrick gets wildly upset about the whole situation: "'Listen,' I say, my voice trembling with emotion, 'have whatever you want but I’m telling you I recommend the Diet Pepsi.' [...] for a moment think I’m going to cry; my chin trembles and I can’t swallow." This hyperbole shows the absurd depths of Patrick's need to aesthetically monitor, critique, and control the world around him. Perhaps Patrick's panic is darkly funny here, but his strongly-held beliefs on how people ought to act are directly connected to Patrick's thoughts on which people do or do not have value, and deserve to live. As a result, this hyperbolic panic gives an unsettling glimpse into how Patrick chooses his murder victims throughout the novel.
In the chapter "Summer," Patrick and Evelyn agree to take a vacation to the Hamptons. Patrick uses an obscure simile to describe Evelyn's reaction to his suggestion:
My nightly bloodlust overflowed into my days and I had to leave the city. My mask of sanity was a victim of impending slippage. This was the bone season for me and I needed a vacation. I needed to go to the Hamptons.
I suggested this to Evelyn and, like a spider, she accepted.
Evelyn accepts "like a spider"—what does Patrick mean by this? Patrick says that he is in a "stupor" earlier in this chapter, which certainly comes through here: it is not at all clear what Patrick means in comparing Evelyn to a spider. Patrick does not discuss spiders anywhere else in the book, and for all of Patrick's anxieties and irregular behaviors, he never shows any particular aversion or affection to bugs. (Certainly one can imagine there are not many bugs in Patrick's sterile, high-rise apartment.)
Patrick might be implying that he is some kind of prey caught in a web, the fly to Evelyn's spider, that she is somehow trapping him in the trip to the Hamptons. But Patrick is the one that suggests the trip to her, and anyway, this feeling would be totally incongruous with Patrick's usual controlling feelings toward Evelyn and toward women in general. Perhaps the best interpretation is that Patrick is simply entirely deranged in this moment: his "mask of sanity" has slipped so much that he can no longer narrate the book coherently. At this point in the novel, Patrick's mental state has bled into the prose, this simile representing Patrick's inability to accurately interpret his surroundings.