You've doubtless used dictionaries before, but the OED takes dictionaries to a new level. The OED's not just about definitions, though of course it has those, too. It's a historical dictionary — in fact, the original title was A New English Dictionary on Historical Principles.
That means that it gives you useful information about the history of each word: where it came from, who first used it in each sense, which important writers have used it in which senses, when senses have become obsolete, and so on. So when you use it, take advantage of the historical aspect. Check, for instance, to see if a word has picked up new meanings around the time it's being used — that tells you that something important is going on in the culture of the time.
Is your author the first one to use the word in that sense? (The OED tries to record the first use of every sense of every word, and though of course it can't always be accurate, it's a good guide.) Or perhaps he or she was the last to use a particular sense?
Are there meanings lurking in the etymology? Every good dictionary provides information about where a word comes from, its root in Germanic languages, in Latin, in French, or in more exotic languages. And sometimes these words contain important ideas that aren't obvious on the surface. The word ardent, for instance, means something like “passionate, enthusiastic, fervent.” But it comes from the Latin word ardere, “to burn.” So when Frankenstein's Creature says at the end of the novel,
Think not, Walton, that in the last moments of my existence I feel that burning hatred, and ardent desire of revenge I once expressed; but I feel myself justified in desiring the death of my adversary,the word ardent combines with burning to reinforce the Creature's plan to immolate himself at the North Pole.