This was the first film that Robert De Niro did after Raging Bull (1980). He kept some of the weight that he gained for Raging Bull because he felt that it better fit the character, although he still had to drop 25 kg (55 pounds).
To prepare for his role as Detective Tom Spellacy, actor Robert Duvall went on the night-beat with real-life Los Angeles area homicide detectives. Duvall also went on a stake-out, witnessed the administration of a lie-detector test and visited a real murder victim crime scene.
The movie is based on the real unsolved murder of Elizabeth Short who was known as the Black Dahlia. The film used facts from this case as story elements. The Black Dahlia murder case made media headlines in 1947, this film was made and released about thirty-four years after this.
The film's marketing boasted the star-teaming of the two ROBERTs - 'Robert de Niro' and Robert Duvall. The film's marketing also emphasized alliteration of their last names: "DE NIRO --- DUVALL. True Confessions." De Niro and Duvall had been rivals for the Best Actor Oscar at 1981's Academy Awards just six months before the film launched stateside. Duvall had been up for The Great Santini (1979) but de Niro won for Raging Bull (1980). Neither were Best Actor Oscar nominated for this movie as expected by industry insiders but the two were jointly awarded the Best Actor Golden Phoenix Award for their performances in this film. Duvall and de Niro had both previously appeared in The Godfather: Part II (1974) but they never appeared together in the same scene but finally in this movie they did.
The moment Robert De Niro and Robert Duvall were cast it was also announced that Bill Conti was going to score the film. Since the production and post production schedules were extended because of the lengthy shoot, Conti had to leave the project due to scoring the James Bond film, For Your Eyes Only (1981)
The film was originally scheduled to open in late 1980 in time, but the post production schedule was extended because United Artists didn't want the film to compete against its Oscar contender Raging Bull (1980) which also starred Robert De Niro and had settled on a release date of February 1981 but was finally pushed back to September 1981.