What is Biff's big secret that he finally reveals to Willy? (one big incident, and the ongoing ones)
I believe you are referring to the Oliver incident in Act 2. Biff's "big secret" is that he stole a pen from Bill Oliver. It may seem a minor incident, but the desperately symbolic gesture holds a world of meaning.
The senseless theft finally makes Biff realize the truth: that he had never, ever been a salesman for Oliver, only a shipping clerk. Biff's...
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