At the games which coincide with the Feast of Lupercal, audiences briefly meet Antony, who is a participant in these games and has a brief dialogue with Caesar. As Brutus and Cassius observe, a troubled and conflicted Brutus states, "I am not gamesome. I do lack some part/Of that quick spirit that is in Antony." In this case, Brutus's use of the word "spirit" refers to Antony's energy, liveliness, or drive--characteristics that will be important later in the play.
Later, and once Brutus agrees to join the conspiracy, he and the other conspirators revisit the relationship between Antony and Caesar. It is during this discussion that audiences become aware of the two very different perceptions that Brutus and Cassius have of Antony. While Cassius is wary of him and thinks he will be a threat to the success of the conspiracy:
I think it is not meet
Mark Antony, so well beloved of Caesar,
Should outlive Caesar. We shall find of him
A shrewd contriver; and you know his means,
If he improve them, may well stretch so far
As to annoy us all. Which to prevent,
Let Antony and Caesar fall together.
Brutus, on the other hand, thinks that killing Antony will make the conspirators appear to be cruel murderers:
Our course will seem to 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.
Brutus ends his speech by assuring Cassius that Antony is no threat. He calmly states, "And for Mark Antony, think not of him;/ For he can do no more than Caesar's arm/ When Caesar's head is off."
It is obvious, from Antony's success in turning the Romans against the conspirators during Caesar's funeral speech, that Brutus grossly underestimates Antony's intelligence, shrewdness, and ability to function on his own without Caesar. This error ultimately leads to the ultimate downfalls of both Brutus and Cassius.