Hitchcock was famously known for showing extreme emotion through what is now called "negative acting". It's the opposite of the typical "ZOINKS!" reaction that defined much of the silent era as well as 80s sitcoms ("whatchoo talkin bout willis??"). Suppose you want to show someone's shock at receiving bad news. Instead of showing them jump and bite their lip, Hitchcock would first show them with a happy or carefree expression; then after they receive the bad news their previous expression melts away to something totally neutral. In other words, by suddenly bringing an upbeat mood down to zero, Hitchcock achieves a deeper sense of shock at the lack of mood.

There's a scene in The Great Dictator that, I think, outdoes even Hitchcockian negative acting. It's the scene where Hannah & the Barber escape to the roof amid the stormtroopers' raid on the Ghetto. Once up there, the barber turns his back completely to the camera as he looks ahead at his barbershop burning down. "There goes the barber shop" is all he says, and we don't even see his mouth, just the back of his head completely still. The emphasis comes from Hannah who is shown to us in profile as she looks at the barber. Her expression of deep horror, pity and her attempt to console him by talking about their dream life in another country is met with his silence and complete lack of movement. The scene goes on for about a minute with her talking, him staring, and the shop burning down.

WOW. Everything about that scene is fine tuned for maximum emotional effect. And the funny thing is we don't even see the actor's face!