How does the diary function as a literary device in 1984?
The diary is a symbol for Winston's desire to be free. He is unable to speak his mind, think his true thoughts, or do anything that may mark him as a suspicious character. Even his facial expressions can give him away...he must be in complete control of himself all the time. In the diary, he can write what he is thinking and feeling without the fear of being turned in by someone nearby. Of course, there is always the possibility that the diary will be discovered, but as long as he writes in it where the telescreen can not see him and he keeps it hidden, he can be relatively certain that his thoughts are safe.
It is not until his actions with Julia in the hidden apartment are discovered that the Thought Police search his home and also discover the diary which helps to incriminate Winston.
The diary is a symbol of the freedom he desires. It represents a time when all people, like the Proles, were able to think and express those thoughts freely.
In 1984, Winston's diary functions as a symbol of his hatred for the Party. This is clearly shown by the fact that he writes "DOWN WITH BIG BROTHER" over and over in the diary. For Winston, this is the first opportunity that he has ever had to voice his discontent with Party rule.
In addition, the diary is also symbolic of Winston's desire to have freedom of expression. One of Winston's diary entries, for example, reads like a stream of consciousness. It lacks organization and punctuation. It is, quite literally, a flow of words as they come to him. For Winston, then, the diary is symbolic of his desire to freely speak his mind, without fear of being arrested and tortured.
The diary also acts as a metaphor for Winston's repressed feelings of rebellion. Just as Winston must hide the diary so that it is never discovered by the Party, so too must he hide his true feelings about what it is really like to live under Party rule.
There is irony in Winston's writing in his diary. There is little doubt that he wrote even one word in the book without someone knowing about it; although this is not known to us when the book starts, we soon realize it must be true.
In terms of the structure of the novel, Orwell uses the diary to involve us in the plot (he does this with another book later in the book when he needs/wishes to convey information [lecture us] about the Emmanuel Goldstein text). The diary also functions to engage us in Winston's character. We know that he is not just "another" party member, that he is up to something that we want to know more about. At the very beginning of the book it gives us some hope that we have here a man whose life may be important; once we learn more about their society, we know that it seals his doom.