By keeping himself in the dark with regards to Infinity War’s script and story, Stan can also keep Bucky in a similar headspace, and trust the film’s directors to maintain the ship’s course.
Bucky has so many internal issues that he has to focus on at the moment that it would almost feel unconvincing for him to have a firm grasp on every nitty-gritty detail of Thanos’ invasion.
As a former cryogenically frozen assassin who can be turned on simply by stating a few code words, not knowing everything about Infinity War actually seems to help Sebastian Stan perform as Bucky Barnes in the MCU.