It may seem impossible for an actor to play a role on camera while having no clue what the movie they're working on is about, but money makes people do funny things.
Actors often receive scripts and don't bother to read the whole thing or ask the producers what their character and scene are all about, which leads to a "WTF am I doing in this movie?!" moment.
Judi Dench was completely lost while playing Aereon in The Chronicles of Riddick, and when she was asked about the film in 2015 she replied ""I've never watched it and I don't think many other people have."
That kind of cavalier attitude is nothing new in Hollywood, and actors have appeared in movies they hate simply for the money from the beginning, even if that hated film is later seen as a classic.
British actor Leslie Howard played Ashley Wilkes in Gone With The Wind, a role he despised in a movie he hated so much that he never bothered to watch it in it's entirety.
But Leslie didn't endure that cinematic tripe for the money- he was offered the opportunity to produce another movie if he would just suffer through a "terrible lot of nonsense" called Gone With The Wind.