According to an interview with Director Richard Donner on the DVD of Payback: Straight Up-Director's Cut (2006) that he met writer Brian Helgeland when he was driving at the Warner Bros. Gate and saw Brian holding a sign that said "Will Write For Work, For Money". Richard got out of the car and asked him about the sign, Brian replied that he was screenwriter and was looking for work. Donner had decided to give Helgeland a chance which led to the two of them working on this film. Helgeland has also worked with Donner on Assassins (1995).
When Jerry is in the hospital for the first time (where he writes the word Geronimo on the wall), the room number on the hospital door is briefly visible: 322. This number is well-known in conspiracy circles as being the address of the Skull & Bones society at Yale University, which is said to have the skull of Geronimo in its possession.
In the movie several times an earthquake is referenced that took place in southern coast with a magnitude of 7.3 during a presidential visit. There indeed happened a 7.3 earthquake in North-West part of Turkey in 1999, just to be followed by a presidential visit.
The doctor who lets Julia Roberts into the restricted area to see Mel Gibson handcuffed to the bed is Gibson's younger brother Donal Gibson. He also comes in to help Gibson when he is suffering a heart attack.
The film makes references to Kennedy assassin Lee Harvey Oswald. In the film, Jerry evades capture by running from a Barnes and Noble bookstore and seeks sanctuary in a theater. This is a reference to Oswald, who ran from a book depository and hid in a theater after the assassination.
Jerry's cab is numbered 1S48. The bus that Jerry hooks up to the fruit cart is 1648. This is a tribute to stuntman Harry Wowchuk who worked on this movie and who was born on October 16, 1948 and who has worked with Richard Donner and Joel Silver on three movies.
When Jerry and Alice are at Jerry's apartment, he makes a mention of an Oliver Stone and President Bush connection, calling him "their spokesman". 11 years later, Stone directs 'W. (2008/I)', a biographic film on the life of former president George W. Bush, son of also-former president George Bush.
When Alice Sutton (Julia Roberts) returns to Jerry Fletcher's torched apartment, looking for clues on his conspiracy wall, the mugshot of Ezekiel Walters is played by a shot of Brian Helgeland, the writer.
All of the scenes at the horse farm were shot at Lionshare Farm in Greenwich, CT, which is owned by United States Equestrian Team member Peter Leone. Leone coached Julia Roberts through the scene at the end where she rides her horse across a field at a gallop.
(at around 1h 33 mins) A truck is seen with the logo "Hynek Enterprises, Inc." This is a reference to Dr. Allen J. Hynek, astronomer and scientific advisor to the Air Force's UFO research programs in the 1940s, 50s, and 60s.