Based on the real Robert Graysmith, whose books formed the basis for the film. Jake Gyllenhaal portrays his descent into obsession with the case accurately, according to Graysmith, though his direct involvement and relationships with others are dramatized.

Based on the real Paul Avery. Robert Downey Jr. captures his flamboyant personality and involvement in reporting on the Zodiac case, including receiving direct threats, though his close investigative partnership with Graysmith is largely fictionalized.

Based on the real Dave Toschi. Mark Ruffalo accurately portrays him as the persistent lead detective on the case in San Francisco, facing frustration and bureaucratic hurdles. Toschi was a consultant on the film.

Based on the real William Armstrong. Anthony Edwards accurately portrays Toschi's partner during the height of the Zodiac investigation.

Based on the real Arthur Leigh Allen, a long-time prime suspect in the case. John Carroll Lynch's portrayal reflects Allen's appearance and known circumstantial evidence against him, as presented in Graysmith's books.

Based on the real Melvin Belli, who received communication (purportedly) from the Zodiac, including a famous call to a TV show while Belli was a guest.

The film's recreations of the attacks at Blue Rock Springs (Ferrin/Mageau), Lake Berryessa (Shepard/Hartnell), and Presidio Heights (Stine) are meticulously detailed and based closely on survivor testimony and police reports.

The killer's campaign of sending taunting letters, coded messages, and pieces of evidence (like Stine's shirt) to Bay Area newspapers (primarily the Chronicle) is accurately depicted.

The film accurately shows the first complex cipher being published and solved not by authorities, but by Donald and Bettye Harden, a Salinas schoolteacher couple.

Accurately portrays the difficulties faced by investigators due to the crimes occurring in different counties (Vallejo, Napa, San Francisco), leading to communication issues and jurisdictional friction.

The newspaper's central role in receiving and publishing Zodiac's letters, and the involvement of journalists like Paul Avery, is accurately depicted.

Based on Graysmith's own account of becoming consumed by the case and conducting his own research after official investigations stalled. The extent of his direct interaction with Avery/Toschi during this period is dramatized.

Allen was genuinely a significant suspect investigated by police. The film accurately presents the circumstantial evidence pointing towards him (watch, boots, statements, etc.) as detailed in Graysmith's book.

Accurately reflects that despite strong circumstantial evidence, investigators could never find definitive physical proof (fingerprints, conclusive handwriting match, DNA) linking Allen to the Zodiac crimes.

The film depicts them as close colleagues working together extensively on the case. In reality, Graysmith and Avery barely knew each other during this period; their close working relationship is a fictional element.

The Zodiac did make contact, including a phone call (possibly an imposter) to lawyer Melvin Belli during a live TV talk show appearance Belli made, as shown.

Zodiac did make a credible threat to attack a school bus, causing widespread fear and police protection details, as depicted.

Allen was interviewed by police multiple times, and his residence was searched, based on circumstantial evidence and tips, as shown in the film.

Based on Graysmith's account of encountering Allen at his workplace. The tense, silent stare-down depicted is a dramatization of that encounter.

Survivor Mike Mageau did identify Allen from a photo lineup in 1991, years after the attack. However, police largely discounted this identification due to the time lapse and Mageau's initial uncertainty. The film presents it as a more definitive final moment.

The film accurately concludes by stating that the Zodiac murders remain one of America's most famous unsolved cases, with no one ever definitively charged.

Meticulously recreates the look and feel of various Bay Area locations (San Francisco, Vallejo, Napa) during the relevant time periods, capturing the specific atmosphere of Northern California.

Accurately portrays the bustling, smoky environment of a major newspaper office in the 1970s, including period-appropriate technology, desks, and workflow.

Realistically depicts the look of police stations, interrogation rooms, and the procedural aspects (paperwork, jurisdictional issues, evidence handling) of police work during the era.

The specific locations of the Zodiac attacks (e.g., Lake Herman Road, Blue Rock Springs park, Lake Berryessa shoreline, Presidio Heights street) are recreated with attention to geographical and atmospheric detail.

High degree of accuracy in costumes, props, vehicles, and general production design reflecting the changing styles across the decades covered in the film.

Effectively conveys the public anxiety generated by the Zodiac's reign of terror and the intense media focus (and the killer's manipulation of it) surrounding the case.

Accurately portrays the communication and investigative tools available at the time – rotary phones, typewriters, physical case files, hand-drawn ciphers – highlighting the pre-digital nature of the investigation.