Richard Stieg was a research assistant in the weapons systems research division of Massive Dynamic. At an unknown time, he was contacted by unknown persons and asked to steal research into airborne viral contagions - research that had been conducted by Walter Bishop and William Bell decades earlier. Setting up a laboratory in the U-Case Storage facility in Back Bay, Massachusetts, Stieg was able to manufacture a Flesh-melting contagion which causes the infected individual's skin to decompose in a matter of seconds. The contagion was then smuggled onto Flight 627 in his twin brother Morgan Steig's insulin pen. It is unknown whether or not Morgan knew he was carrying the contagion onto the plane.
Following the Flight 627 incident, Richard Stieg was threatened by John Scott. Stieg attempted bravado, reminding Scott of Morgan's fate, but Scott's reply implied that Stieg was just as disposable as his brother. Following this exchange he appears at Logan airport, where the plane landed, but is denied entry at the gate.
Scott and Olivia Dunham eventually tracked Stieg down at the U-Case facility. Stieg attempted to flee and detonated the garages surrounding Scott, exposing him to the raw ingredients of the Flight 627 contagion. Stieg was able to escape. Olivia later participated in a Synaptic Transfer with Scott, allowing her to access his memories and identify Stieg's face. The FBI discovered that Massive Dynamics was his last employer, and Nina Sharp gave Olivia his address. The FBI raided his apartment, finding equipment similar to the labs set up at the U-Case facility. Stieg escaped but was chased by Olivia and Peter Bishop, eventually being wrestled to the ground by Peter. He gave Olivia the complete list of the ingredients to the contagion, allowing Walter to engineer a cure for Scott. As Scott recovered, Stieg told Olivia that he was threatened, but does not reveal by whom, and tells her where he has buried a recording of their telephone conversation. While Olivia was out looking for the tape, Scott broke into Stieg's room and suffocated him with a pillow. ("Pilot")
- Richard Stieg's DOJ file lists his address as: 629 Easter Ave., Boston, Massachusetts. This address does not exist in real life, however 629 Eastern Ave is a public storage facility.
- "Steig" translates from German as "Path."