In the scene when Jim (Christian Bale) and Mike ('Freddie Rodriguez') visit Darrell (Terry Crews), after shooting the scripted material, they found they still had the location for two hours, so they began to improvise. They ended up with a thirty minute scene of the three of them talking about their lives in-character. According to Christian Bale, it was one of the funniest experiences of his career.

David Ayer first met Christian Bale when Bale auditioned for Training Day (2001), for the role that ultimately went to Ethan Hawke. Ayer liked Bale's intensity, and gave him the 'Harsh Times' script. Bale loved it, and Ayer promised him that if the movie was ever made, he could play Jim. When the movie went into production, Ayer kept his promise.

According to David Ayer, the script for Harsh Times (2005) was written in 1996. He wrote this script shortly after he wrote the script for Training Day (2001), which was completed a year earlier. The script is semi-autobiographical and is based on his own experiences. Like the character of Jim (Christian Bale), Ayer grew up in South Central LA, and joined the military. Upon returning to LA, he too found himself drifting into petty crime, and hanging out with his friend all day. He was able to pull his life together however, but some of his friends were unable to do so. As such, the character of Jim is a composite of both Ayer and a number of people he knew.

Christian Bale spent three weeks with gang members, police officers, ex-soldiers and Homeland Security agents prior to the commencement of shooting. Bale originally wanted to do a full Ranger's School course, which takes eight weeks, and has a 70% drop out rate, but the shooting schedule wouldn't afford him enough time, so he was given a crash course over a week instead.

The gun that Jim is seen carrying at the end of the film had to be rented by a Mexican police officer as no firearms were allowed over the Mexican border.

Freddy Rodríguez worked on this film at the same time as he was shooting season 5 of Six Feet Under (2001). He would work on the show on Saturday, Sunday and Monday, and on the film on Tuesday, Wednesday, Thursday and Friday.

David Ayer had to take out a mortgage on his home to finance the project.

The script was initially developed as a Sundance Institute Screenwriters Lab Script, where a live read through in front of 350 people was done. Their positive reaction prompted Ayer to try to make the movie.

The word 'fuck' and its derivatives are used a total of 296 times in this film, an average of 2.47 times a minute.

The entire film was shot in 24 days.

The shack where Marta (Tammy Trull) lives was a real shack, not one built for the production, in a town situated just south of Ensenada.

According to Eva Longoria, money was so tight on the film, the production couldn't afford catering.

The license plate 2GAT123, which appears in this film, has also featured in many other films, including Beverly Hills Cop II (1987), Traffic (2000), Pay It Forward (2000), Mulholland Drive (2001), Crazy/Beautiful (2001), L.A. Story (1991), Two and a Half Men (2003) and S.W.A.T. (2003) (which was also written by David Ayer).

Emilio Rivera and Paul Renteria (the man who stabs Eddy), are good friends in real life. They also worked behind the scenes as dialect and culture coaches for the Welsh-born Christian Bale and the Chicago-born, Puerto Rican Freddy Rodríguez. Rivera and Renteria had worked together in the early 1990's in the Historic Chicano Play "The Silver Dollar" performed in front of live patrons in the Silver Dollar Bar on the famous Whittier Boulevard in East LA, which commemorated the shooting and actual death site of LA Times Correspondent 'Rubén Salazar' on August 29, 1970, during the East LA Riots. They also appeared together in 2007's Toyota Tundra "Band Roadies" Spanish Television Commercial & Latino Concert Series. Off screen, they speak to High School students about Acting & Life Issues, participate in Celebrity Softball Games, and can often be seen on the streets of Los Angeles supporting Children's Toy Runs & Various Fundraisers on their Harley-Davidson Motorcycles.

Both Harsh Times (2005) and Batman Begins (2005) open up with a nightmare sequence going on in the head of the main character, both played by Christian Bale.

Freddie Rodriguez's character's last name is Alonzo. In Training Day, which David Ayer also wrote, Denzel's first name is also Alonzo.

Noel Gugliemi, who plays Flaco, wears a jacket with a name tag that says 'Hector'. In The Fast and the Furious (2001), Noel Gugliemi played a character called Hector.

Christian Bale and Freddy Rodriguez played DC characters. Bale played Batman in The Dark Knight trilogy, and Rodriguez voiced Mas y Menos in Teen Titans.

The film was released by Bauer Martinez Entertainment, however this was not the first company writer/director David Ayer went to. Ayer had originally sold the script to another company, but they demanded changes that Ayer was unwilling to implement. They wanted several flashback scenes of Jim (Christian Bale) and Mike (Freddy Rodríguez) as kids, to help establish their bond, they wanted less profanity, and they also wanted the ending of the film changed, so that Jim survives, and puts his life back together. Ayer refused to make the changes, and purchased the script back.