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.