As in any literary work, dialogue serves to advance the narrative and develop the characters. In Animal Farm these objectives are accomplished along with the use of dialogue to advance propaganda. In addition, the voices in dialogue serve to develop Orwell's satire of totalitarian government.
In the opening chapter, Old Major communicates his ideology to the other animals. He argues,
Man is the only real enemy we have....All men are enemies. All animals are comrades.
However, it is not long before the verity of his pronouncement is shaken as Napoleon and Snowball become engaged in contentious debate. However, Snowball often wins over the majority with his speeches. He "talked learnedly about drains, silage and other relative things. It is always the pigs who dictate the rules. When there is discontent, Squealer is sent to make the necessary explanations and propganda.
As time goes on, Napoleon becomes very good at discrediting the benefits of the windmill that Snowball has suggested. Together with his propagandist Sqiealer, Napoleon is effective in disparaging Mr. Hutton and all others who stand in his path to dicatorship, while at the same time exposes problems with communisn.