O'Bannon and Shusett divided the script into three acts, and identified the main story conflict as "a crazy psychologist is trying to kill one of his patients". They realized the script only addressed this conflict in the third act, in which the hero also defeats the villain. They took out the first such scene, in which the conflict first matures, and moved it to the end of the first act.
Then, O'Bannon wrote a completely new scene to the end of the second act, in which the main characters suddenly freeze into a showdown. Both writers felt the revisions made the script more interesting; O'Bannon compared the rewrite to "a watchable sow's ear". Shusett sold the script to a Canadian producer, who attached John Huston to direct.
A further production draft was written by the producer himself. Shusett read the new draft and disliked it. He called Huston up to offer their earlier draft instead. Huston opted to use the producer's draft as a matter of convenience. (O'Bannon, Lohr: "Guide to Screenplay Structure". 2013, Michael Weise productions, pp.8-9)