At different points in the novel, Napoleon joins the other farmers. He begins to engage in trade with them in chapter 6. Although the animals don't know what kind of trade, they have already seen the results in dwindling food supplies. The animals' health and well-being continues to suffer. Napoleon, the great leader of the animals, has obviously broken one of the great Animal Commandments and is engaging with humans, but his mouthpiece Squealer helps the animals understand that it has to be done.
In chapter 10, Napoleon really joins the other farmers. He has them over for a meal and as the animals watch the great pig from the window of the farmhouse, they see him drinking and enjoying the company of the other humans. Enjoyment turns to anger as more and more alcohol is consumed:
Twelve voices were shouting in anger, and they were all alike. No question, now, what had happened to the faces of the pigs. The creatures outside looked from pig to man, and from man to pig, and from pig to man again; but already it was impossible to say which was which.
The pigs had turned into humans.