In Othello, when does Emilia choose to be silent and when does she choose to speak up?

Emilia is silent when Desdemona wonders aloud where her
handkerchief is, and Othello questions Desdemona about it.  Emilia knows that she has
stolen the handkerchief and given it to Iago.  But she has no idea what Iago is planning
to do with it.  When Desdemona worries about the lost handkerchief, Emilia probably
considers it a "trifle" and remains silent (Act 3, scene 4). In Act 5, it is Emilia's
revelation that she was the one who stole the handkerchief at Iagoa's request that
unravels Iago's scheme and exposes Iago for the villain that he is.  Throughout the
play, Iago has cautioned Emilia to hold her tongue.  At the end of the play, Emilia
disobeys him.  This disobedience costs her her life, but she does not die in
vain.