Could have gotten away with? Sure. David Prowse had no idea that Darth Vader was Luke's father when filming the scene where Vader tells him exactly that (instead having been told the key line was 'Obi-Wan killed your father.') But if you want this detail to in any way inform how the actor behaves towards his 'son', then it's a good idea to tell him. Whether or not Sean Bean had no subtelty in his Ned Stark performance is hindsight. I thought the show handled it fairly well - Ned Stark is shown being very recitent when talking about Jon's mother with Robert Baratheon, for example.