I have seen a panel video where I think it's Jensen says that they almost never stop rolling the cameras because that's where they get all their good blooper reel footage, and even something they can put in the episode.
(gif set) "Can we talk about Sam's face in the last gif? His fucking smile? When is the last time he smiled that honestly? And it's all because Bobby says Dean is adorable, and Sam just breaks out in this unabashed grin and look at how happy everyone is in this moment, okay?! Why don't we have moments like this anymore?!" "Because Bobby died." "GO SIT IN THE CORNER." ||| Supernatural 4x06 "Yellow Fever"