While filming, Will Ferrell wore an earpiece that fed him Dame Emma Thompson's narrative lines, in order to assist the other cast members in reacting more naturally to Ferrell's seemingly non-sequitur lines.
A single page of the book can be glimpsed if this movie is paused, while Professor Jules Hilbert (Dustin Hoffman) is reading it. The page quotes word-for-word the opening narration of this movie 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.
When Karen (Dame 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 23-year-old Evelyn McHale leapt to her death from the 86th 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.
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 of 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 amongst mathematicians and philosophers, and several are still unanswered.
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 - Odyssee im Weltraum (1968). It is speculated that these characters can possibly be the "heroes" of the writer.