After Brutus allows the conspirators into his house in Act 2, Scene 1, he almost immediately begins asserting his opinion on how the assassination should be carried out (even though he is the last to join the group). First, Brutus orders the others to come forward and shake hands with him. Cassius suggests not simply shaking hands but also swearing an oath to one another, and Brutus quickly replies,
"No, not an oath" (2.1.115).
Brutus goes on to explain that if the times they are living in are not enough to keep them faithful to their cause then they should just forget about the plan and go back to bed.
Brutus's second change is that the group should not include the respected, older senator, Cicero. While Metellus believes that Cicero's involvement in their plan would strengthen their support after the assassination, Brutus does not want to tell Cicero of their plot because Cicero will not follow what other men have initiated. He is too much of a leader to do so.
The most significant part of the planning that Brutus changes is to not kill Antony along with Caesar. The other conspirators believe that Antony will be too strong a force to be reckoned with if they let him live, but Brutus argues that
"Our course will seem too bloody, Caius Cassius, / To cut the head off and then hack the limbs, / Like wrath in death and envy afterwards, / For Antony is but a limb of Caesar" (2.1.164-167).
His words demonstrate that he is concerned about their reputations after the assassination and that he thinks that Antony will be powerless after his head (Caesar) is removed. This decision on Brutus's part is, of course, pivotal in the play's action.
What readers should note from this scene is that Brutus disagrees with virtually every suggestion made by Cassius. Cassius, once the plot's ringleader, relinquishes leadership to Brutus, and in doing so, seals his own fate.