While filming, Will Ferrell wore an earpiece that fed him Emma Thompson's narrative lines, in order to assist the other cast members in reacting more naturally to Ferrell's seemingly non sequitur lines.
The film borrows heavily from "Niebla" by Miguel de Unamuno, a Spanish novel about a character who becomes aware he is being narrated by a writer and goes to visit him. In de Unamuno's story, however, the main character commits suicide.
A single page of the book can be glimpsed if the film is paused, while Professor Jules Hilbert (Dustin Hoffman) is reading it. The page quotes word-for-word the opening narration of the film as Harold goes about his day. The page also contains a detail that is not mentioned otherwise, Harold's co-worker Diane Gordon has been in love with him since the eighth grade, but is too shy to say so, and in the shown page when Harold requests a file from her, she asks for clarification in the hopes he might once say "good morning" to her.
The last names of all of the characters (and the bus line and publishing firm names) are the names of mathematicians, scientists, engineers, artists, et cetera, (Harold) Francis Crick, with Watson and Wilkins, found the structure of DNA; (Ana) Blaise Pascal, French mathematician and philosopher; (Karen) Gustave Eiffel, engineer and designer of the Eiffel Tower; (Penny) M.C. Escher, Dutch graphic artist; (Dr.) Magnus Gustaf Mittag-Leffler, Swedish mathematician; (Professor Jules) David Hilbert, German mathematician; (Doctor) Gerardus Mercator, sixteenth century Flemish cartographer; (Kronecker Bus Line) Leopold Kronecker, German-born mathematician and logician; (Banneker Press) Benjamin Banneker, free African-American mathematician, astronomer, clockmaker, and publisher; (Dr. Cayly) Arthur Cayley, nineteenth century British mathematician. Even Dave (no last name) seems to be a reference to the main character from 2001: A Space Odyssey (1968). It is speculated that these characters can possibly be the "heroes" of the writer.
Professor Jules Hilbert (Dustin Hoffman) creates a questionnaire of twenty-three items for Harold Crick (Will Ferrell). Professor David Hilbert, the German mathematician, proposed a famous list twenty-three problems at the International Congress of Mathematicians in Paris in 1900. Hilbert's problems became so famous, that they are typically referred to by number among mathematicians and philosophers, and several are still unanswered.
The title of the movie comes from a famous quote from Lord Byron's "Don Juan": "'Tis strange, but true; for truth is always strange; Stranger than fiction: if it could be told, How much would novels gain by the exchange! How differently the world would men behold!"
When Karen (Emma Thompson) first meets Penny (Queen Latifah), she mentions a photograph of a beautiful woman who had committed suicide by jumping from a building. This refers to an actual event, in which twenty-three-year-old Evelyn McHale leapt to her death from the eighty-sixth floor of the Empire State Building on May 1, 1947. A full-page photo of her body was published in LIFE Magazine later that month, which is the photo, to which Karen is referring.
Several references are made to René Magritte's painting "Son of Man", once when he is running to the bus stop with the green apple in his mouth, and again when he is talking to the doctor at the office, sitting in front of a wall painted with clouds.
The novel which Karen Eiffel (Emma Thompson) is writing is called "Death and Taxes". This is a reference to the famous quote, "In this world, nothing can be said to be certain, except death and taxes", written by Benjamin Franklin in a 1789 letter addressed to Jean-Baptiste Leroy.
The watch featured in the movie is a Timex Men's Watch #T56371, Ironman Triathlon 42 Lap Combo Dual Tech, though in the film, the watch's LCD display is CG enhanced to present clearer graphics. The actual watch is a simple nine segment per character LCD alphanumeric mode display with three lines, and some special-indicators.
A recurring theme is how Harold's wristwatch changes his life. There are several scenes in which Harold sits on the bench outside Ana's bakery. There is a large, round window in the wall that resembles a watch face through which Harold looks at Ana.
In an early scene, on-screen graphics appear that resemble an image used to illustrate the golden ratio. In mathematics and the arts, two quantities are in the golden ratio if the ratio between the sum of those quantities and the larger one is the same as the ratio between the larger one and the smaller. This ratio can be expressed as a mathematical constant (1.618...), usually denoted by the Greek letter Phi.
Small math and science references were slipped in, such as references to Euclid Street (a reference to Euclidean Geometry) and the Spoon song "My Mathematical Mind", which plays during a sequence near the end of the film.
When Professor Hilbert is speaking to Harold in his classroom, there is a list on the chalkboard behind him of four characters from "The Alexandria Quartet", a four-book set of novels by Lawrence Durrell, published between 1957 and 1960.
A list on Professor Hilbert's classroom chalkboard contains "Mountolive's ear aches, Liza's blindness, Clea's amputated hand, Leila's smallpox, Justine's stroke, and Pombal's gout". These are many of the primary characters from 'The Alexandria Quartet' of novels by Lawrence Durrell. These four books (Justine, Balthazar, Mountolive, and Clea) tell the basically same story from different points of view and explore relationships of people and ideas and concepts of life and mortality as seen through the theme of modern love.
When Harold Crick (Will Ferrell) sits down on the bus, in which he again encounters baker Ana Pascal (Maggie Gyllenhaal), a building with the word "Drury" can be seen on a sign in the background (on the front wall of the bus). In the popular nursery rhyme, Drury Lane is where the Muffin Man lived. However, Drury Lane is also a theatre in Chicago, where this movie was filmed. While the city is never mentioned, visual clues to the city abound throughout the film. When Harold is sprinting to the first payphone, the sculpture that resides in Daly Plaza can be seen. When he starts to read the first draft of the novel on the bus, one of the Marina City Towers can be seen through the bus window. Other famous Chicago locations, like the John Hancock Building and the Sun Times Building, are visible in various scenes throughout the film.
When Karen Eiffel is leaning on Professor's Hilbert book case, there are several books about cooking that can be seen: "Plain Cooking", "Electric Blender Recipies", "A Year of Diet Desserts", "Cooking and Brownies", and "The Slim Gourmet Book".
Maggie Gyllenhaal appeared in another film where the main character's name was Crick. Waterland (1992) has a main character using the surname of Crick (Tom Crick), played by Jeremy Irons, and that movie was Maggie's film debut.