From Vince Gilligan’s Reddit AMA (Breaking Bad spoilers):

“He loved to tease Aaron Paul mercilessly,” explained Gilligan. “This came about after I told Aaron Paul early in the series that I had planned to kill off his character. From then on, every time a new script came out, Bryan would hurry to read it first so he could put on a sorrowful face and say to Aaron, ‘Gee buddy, I’m so sorry. You’re going out with a bang, at least.’ And Aaron, God bless him, seemed to fall for it every time.”