Patrick Stewart was definitely a decision that was somewhat fan driven. Christian Bale was a name that was a dream casting for fans for quite some time, I think I'd heard Oldman for Gordon a number of times before as well. Spacey for Lex Luthor seemed like it existed before the movie. Downey Jr. was a dream by fans for a while too. You can't say how much or how little we influence execs, but don't think for a second they completely tune us out. Sometimes the voices of fan boards are heard if they are loud enough.