What event happens in the antecedent action of Act 1?

In the antecedent action of Act 1 of Shakespeare's Julius Caesar, Brutus is approached by Cassius, a close friend and fellow Roman senator, who expresses concern about Caesar's growing power and ambitions. Cassius fears that Caesar may become a dictator and suggests that Brutus, being held in high esteem by the people of Rome, should join him in a conspiracy to assassinate Caesar.