Winston also speaks in passive voice. Most teachers will encourage students NOT to do this as it is less engaging, less interesting, and tends to bog one down in the reading. However, for this novel it is perfectly suited for Orwell's purpose. He absolutely wants us to FEEL the oppression Winston, Julia, and others like them are feeling. With passive voice, Orwell succeeds. The action is being done to the characters in the novel...they are just along for the ride (this is OK for the not-so-intelligent ones who are satisfied living the way someone else dictates, but not for the ones who are still capable of thinking and feeling for themselves)...not in control at all, rather at the mercy of the government and Big Brother. Perfect!
An interesting and complex question. For all that he is in great pain at times, the tone in Winston's sections of the novel are distant, almost objective and flat, as if Winston is simply observing what happens to him, even when it is intense. This combination of factors makes the diction very precise.
However, at times the tone grows painfully ironic, as when O'Brien is "re-educating" Winston. The gap between the torture and the relatively calm words creates the irony, and the pain. The diction here is just as precise, but the gap between words and reality creates new emotional realities.
When the philosophical background of the Party is discussed, the tone is deliberately confusing. That's one point of Newspeak.