Animal Farm hinges on Orwell's ability to portray characters that compare to those whom he was criticizing from the Russian Revolution. Squealer plays a pivotal role in the novel with his ability to turn "black into white."
The first real show of power from the pigs is when the animals fail to stop the pigs taking the milk for themselves. Squealer is instrumental in convincing the animals that it is the best thing for everyone. The animals are so afraid of returning to a life under Jones that even a ridiculous claim that it has been
scientifically proven that milk and apples are essential to the well-being of a pig
is accepted. The disappearing milk reveals the inequality that exists when "All animals are equal but some are more equal than others."
The simplistic nature of the sheep allows Squealer to take full advantage--over simplifying language as in "four legs good, two legs bad," which will change without the sheep understanding the significance of the change and over- complicating language. For instance, "a bird's wing....is an organ of propulsion" seen later in the book confuses and intimidates the sheep into blind acceptance.
Squealer's deceitfulness and his loyalty to Napoleon allow the pigs to gradually establish their tyranny. Having no conscience allows Squealer to manipulate the animals who, despite Snowball's contribution to the farm, chant and interrupt Snowball's speeches sufficiently to allow Napoleon to act, eventually running Snowball off the farm when it seems that Snowball's ideals may win over the animals. Fear is used as the dogs chase Snowball.
Squealer, following Snowball's expulsion, uses the animals' fear to again convince them that Napoleon is the best leader to ensure that they don't make "wrong decisions." As a master manipulator and propaganda chief, Squealer basically convinces the animals that the events that took place are not as they remember - Snowball was not a hero, Napoleon was not against the building of the windmill and
the unselfish Napoleon is doing it all for the greater good of the farm.