McCabe said in his book that Mueller is hyper-serious and doesn’t mince words. Said that his chief of staff would tell subordinates to not disturb Mueller on plane rides. He would just read work materials the whole time. Basically, don’t speak unless spoken to. 

He also said that when Mueller made him in charge of a new unit, McCabe told him he would prefer to be working in counterintelligence. Mueller said something like, “Well, I’d prefer to be prosecuting homicide cases, but here we are.” That’s about as casual as the man gets.