The script originally has no back story about Robert, so Denzel Washington contributed much to the character's background and back story including Robert having obsessive-compulsive disorder (OCD). In addition to his daily physical and fight training before filming, Washington met and interviewed several real-life OCD people as to gain insights of how to play that disorder correctly.
The film languished in development hell for several years before it finally began. It was originally developed by the Weinstein brothers in 2005. They brought in novelists Michael Connelly and Terill Lee to write the first script with Paul McGuigan to direct. With no progress, in 2010, the rights were then sold to Mace Neufeld and Escape Artists'. Paul Haggis was brought in to write another draft with Russell Crowe to play Robert. However other project commitments resulted in both men dropping out before Denzel Washington saw the script and expressed interest in playing Robert.
Nicolas Winding Refn was the original director but left the project one month later in January 2013 when he could not agree to contractual terms with the producers on his deal. Rupert Wyatt was the next person approached, but he declined citing scheduling problems (Wyatt also turned down directing Dawn of the Planet of the Apes for the same reason.) before they finally offered it to Antoine Fuqua.
During the scene in which Robert and Teri are discussing books at the diner, Robert mentions that he is attempting to make his way through the "100 Books Everybody Should Read" list, because it was something his wife also attempted to do before she died. This list contains a book called "Eugene Onegin" by Alexander Pushkin. There is a character by the name of "Pushkin" who Robert later identifies as the "head of the snake," basically meaning the first generation villain of the entire Russian operation that which Robert attempts to derail throughout the progression of the film. Pushkin is, more or less, the final target Robert needs to eliminate in order to complete this particular mission of "equalization".
As well as appearing in "Flight" with Denzel Washington, Melissa Leo also appeared in the original Equalizer television series (Season 1, Ep. 3 "The Defector") where she portrays the daughter of a former Soviet agent who enlists McCall's help to defect to the US.
When McCall contacts the FBI after he takes over the money room and then leaves Detective Masters for the Boston PD, he asks for Agent Mosely. This is an easter egg to the Robert DeNiro, Charles Grodin movie Midnight Run. FBI Agent Mosely was played by Yaphett Kotto.
Several aerial shots of the Leonard P. Zakim [Bunker Hill Memorial] bridge are seen throughout the film. The bridge is the main entry/exit route into Boston, and is featured in several movies and TV shows that take place in Boston.
When Teddy is picked up on the private airport runway by Detectives Masters, Pederson & Remar, Pederson introduces himself as Morgan Pederson. Pederson is played by James Wilcox who has a brother-in-law named Morgan Peterson.
The trivia items below may give away important plot points.
McCall is never actually shown shooting a firearm. At the restaurant, he twists a henchman's arm forcing him to shoot his boss. Towards the end of the film McCall only uses the rifle for the scope and eventually uses a nail gun. Everyone else is neutralized by other creative means.
McCall's former occupation is never fully revealed, but a reference to the "Agency" by Melissa Leo's character which may mean that McCall worked for the CIA in some capacity, most likely in their Special Activities Division, where he would have developed the skills and tactics he used throughout the film.
When his coworkers asked what he did before he worked at the Home Mart, McCall says he was a 'Pip' (as in Gladys Knight & The Pips) & he dances a little for them. When Teddy's men take the employees hostage and are getting ready to shoot one of them, the song that McCall plays on the store PA to get their attention is 'Midnight Train to Georgia' .. by Gladys Knight & The Pips.
Near the end, right before Alina catches up with Robert, there is a bus arriving and leaving, and in its window next to the front door there is a yellow-blue ribbon. This could be a token of Ukrainian Revolution of Dignity of 2013/2014 (originally called EuroMaidan), the struggle of Ukrainian people against criminal abuse of power by Moscow-sponsored mob in the government and big business. However, this is more likely to be the 'Boston Strong' ribbon, which became popular after the terrorist attack of Boston Marathon in 2013.
when Washington's character turns the money over to the police, he writes" follow the money" on one of the hundred dollar bills. In the movie Inside man, Washington plays a police officer who gets a clue from the bank robber that says "follow the ring"