The general seems to just suck the life out of the room when he comes in. The description of the first night at Northanger especially reinforces this: The evening passed without any further disturbance, and, in the occasional absence of General Tilney, with much positive cheerfulness."

I have this image of the room being cold and cheerless when he's there, with a thaw occurring when he steps out to give orders to his butler. He returns, and the cold chill returns. He steps out again to answer an urgent note that has arrived by post and everything is cheery again. And so the evening proceeds.