McEwan's earlier work has explored the fragility of existence using a clinical perspective, Hitchens hails him a "chronicler of the physics of every-day life". Saturday explores the feeling of fulfilment in Perowne: he is respected and respectable but not quite at ease, wondering about the luck that has him where he is and others homeless or in menial jobs. The family is materially well-off, with a plush home and a Mercedes, but justifiably so—Perowne and his wife work hard. McEwan tells of his success rate and keeping cool under pressure; there is a trade off, as he and his wife work long hours and need to put their diaries side by side to find time to spend together.
Perowne's composure and success mean the implied violence is in the background. His personal contentment, (at the top of his profession, and "an unashamed beneficiary of the fruits of late capitalism") provides a hopeful side to the book, instead of the unhappiness in contemporary fiction. McEwan's previous novels highlighted the fragility of modern fulfilled life, seemingly minor incidents dramatically upsetting existence. Saturday returns to a theme explored in Atonement, which plotted the disruption of a lie to a middle-class family, and in The Child in Time, where a small child is kidnapped during a day's shopping. This theme is continued in Saturday, a "tautly wound tour-de-force" set in a world where terrorism, war and politics make the news headlines, but the protagonist has to live out this life until he "collides with another fate". In Saturday Perowne's medical knowledge captures the delicate state of humanity better than novelists' imaginations: his acquaintance with death and neurological perspective better capture human frailty.
The burning aeroplane in the book's opening, and the suspicions it immediately arouses, quickly introduces the problems of terrorism and international security. The day's political demonstration and the ubiquity of its news coverage provide background noise to Perowne's day, leading to him to ponder his relationship with these events. Christopher Hitchens pointed out that the novel is set on the "actual day the whole of bien-pensant Britain moved into the streets to jeer at George Bush and Tony Blair" and placed the novel as "unapologetically anchored as it is in the material world and its several discontents". The Economist newspaper set the context as a "world where terrorism and war make headlines, but also filter into the smallest corners of people's lives." McEwan said himself, "The march gathered not far from my house, and it bothered me that so many people seemed so thrilled to be there". The characterisation of Perowne as an intelligent, self-aware man: "..a habitual observer of his own moods' [who] is given to reveries about his mental processes," allows the author to explicitly set out this theme.
"It's an illusion to believe himself active in the story. Does he think he's changing something, watching news programmes, or lying on his back on the sofa on Sunday afternoon, reading more opinion columns of ungrounded certainties, more long articles about what really lies behind this or that development, or what is surely going to happen next, predictions forgotten as soon as they are read, well before events disprove them?"
Physically, Perowne is neither above nor outside the fray but at an angle to it; emotionally his own intelligence makes him apathetic, he can see both sides of the argument, and his beliefs are characterised by a series of hard choices rather than sure certainties.
He is concerned for the fate of Iraqis; through his friendship with an exiled Iraqi professor he learned of the totalitarian side of Saddam Hussein's rule, but also takes seriously his children's concerns about the war. He often plays devil's advocate, being dovish with this American friend, and hawkish with his daughter.
McEwan establishes Perowne as anchored in the real world. Perowne expresses a distaste for some modern literature, puzzled by, even disdaining magical realism:
"What were these authors of reputation doing – grown men and women of the twentieth century – granting supernatural powers to their characters?" Perowne earnestly tried to appreciate fiction, under instruction from his daughter he read both Anna Karenina and Madame Bovary, but could not accept their artificiality, even though they dwelt on detail and ordinariness.
Perowne's dismissive attitude towards literature is directly contrasted with his scientific world-view in his struggle to comprehend the modern world. Perowne explicitly ponders this question, "The times are strange enough. Why make things up?".
Perowne's world view is rebutted by his daughter, Daisy, a young poet. In the book's climax in chapter four, while he struggles to remain calm offering medical solutions to Baxter's illness, she quotes Matthew Arnold's poem Dover Beach, which calls for civilised values in the world, temporarily placating the assailant's violent mood. McEwan described his intention as wanting to "play with this idea, whether we need stories". Brian Bethune interpreted McEwan's approach to Perowne as "mercilessly [mocking] his own protagonist...But Perowne's blind spot [literature] is less an author's little joke than a plea for the saving grace of literature."
Similarly he is irreligious, his work making him aware of the fragility of life and consciousness's reliance on the functioning brain. His morality is nuanced, weighing both sides of an issue. When leaving the confrontation with Baxter, he questions his use of his medical knowledge, even though it was in self-defense, and with genuine Hippocratic feeling. While shopping for his fish supper, he cites scientific research that shows greater consciousness in fish, and wonders whether he should stop eating them. An Iraqi professor he treated has told him of the brutality of Saddam Hussein's rule, but also takes seriously his children's concerns about the war.