Robert Downey Jr., Jake Gyllenhaal and Mark Ruffalo are set to star in the thriller "Zodiac," Variety reports. The film, being directed by David Fincher ("Seven"), is based on the true story of Zodiac, the infamous serial killer who terrorized the San Francisco for 25 years. Gyllenhaal will star as Robert Graysmith, the journalist and author who wrote the books "Zodiac" and "Zodiac Unmasked," upon which the film is based. Mark Ruffalo will take on the role the lead investigator, and Downey Jr. will play a reporter.