Early on in Hidden Figures, when Katherine is about to start working with Harrison's team, Mrs. Mitchell warns her that he's a hard guy to work for, and that she's not likely to last long in the position because he goes through computers [professional mathematicians, in a time when electronic computers were still a brand new thing] very quickly.
Instead, he's shown to be a supremely reasonable guy, the sort of manager we all wish we could work for. He's dedicated to making his team successful, works the long hours of overtime alongside the rest of them when necessary, and goes well out of his way to remove obstacles from Katherine's path. For example, the guy Mitchell described would probably have fired Katherine after her meltdown over bathroom availability; instead, he responds with one of the most awesome scenes in the film, personally fixing the problem in a way that could easily have exposed himself to negative social consequences.
Do we have any explanation of why the Al Harrison that Mrs. Mitchell knows is such a completely different guy than the Al Harrison that Katherine worked for?