Sean Astin's character Bob is certainly one of the best new additions on Stranger Things 2. However, the character was incredibly different in early iterations of the season, changing drastically once Astin came on board the cast.
As it stands now, Bob is this season's loveable goof. He's a nerd, he's super happy, and in the end, he's incredibly heroic by saving everyone stuck in the lab. Unfortunately, he's also the only major character to die in the season.
That was always the plan. Bob was always going to die, but originally he was going to die in episode three, not episode eight.
"In an early, early outline, Evil Will was going to kill Bob," said producer Shawn Levy on the aftershow Beyond Stranger Things. It would have happened in episode three, "The Pollywog", when Bob gave Will the advice to stand up to his fears. Will was going to pretty much reject that and use his connection to the Mind Flayer to kill Bob.
"He wasn't originally intended to be as big of a role as he ended [up being]," Ross Duffer said on the show. "And then Sean came into the picture and was just so much more than we ever expected."
So everything that's great about Bob: His quirkiness, his heroic ending, his helping everyone out, solving puzzles, it was all added once Astin came on board.
However, that also changed how the creators dealt with Will's arc. If Will had murdered Bob that early, his descent into possession would have been completely different. He may have even been the primary adversary for the whole season. This version, though, Will struggling with the Mind Flayer until the end, is obviously better.
Stranger Things 2 is on Netflix now.