Julius Caesar is quite a macho play, with characters constantly examining their actions in light of their relationship to accepted ideas of manly virtue and strength. Rome is an Empire (though it is not yet ruled by an Emperor), militaristically maintained, and the model of the "good soldier" extends to the citizen and politician as well. Although there's lots of violence in the play, it's not only physical strength and fighting ability that constitute manliness: many characters feel compelled to mask any traditionally "weak" emotions, like fear and sadness, as well as their personal desires and, to an extent, free will. Brutus, for instance, feels compelled to give way to the logic that demands Caesar's death, even though he loves Caesar and is repelled by the idea emotionally. Caesar himself must go to the Capitol even though he suspects his approaching murder, because Caesar feels he must be unwavering, and because death "will come when it will come."
The willingness to abandon self-interest, to brave pain and death for the good of Rome, or to avoid dishonor, is essential to gaining respect. This "virtue" is what demands Brutus's initial complicity in the plot, and his eventual suicide. Portia, as well, ashamed of her female identity, stabs herself in the thigh to prove she can be trusted, and eventually kills herself in the most painful way she can imagine. Ironically, it is the least "manly" of the major characters—Antony, who loves art and parties, weeps openly during his eulogy for Caesar, and symbolically appears naked in his first scene—who emerges victorious at the play's end (though his emotional nature will be his undoing in Antony and Cleopatra, Shakespeare's "sequel" to Julius Caesar).
Though there is certainly violence in Julius Caesar, characters spend far more time talking to one another than they do fighting or killing, and much of that talk takes the form of argument and debate. But unlike the arguments we are used to, those in Caesar focus primarily on discerning what is right—what should or must be done—rather than on characters trying to get their way. In Rome, accusing someone of acting in his self-interest, rather than for the good of Rome, is a serious insult. Though ideally this process should involve logic alone, certain characters in Caesar—just as in life—are skilled at manipulating language to make something seem logical when it is not.
This difference materializes most clearly in the arguments between Brutus and Cassius. Brutus—who reluctantly concludes that he must kill Caesar—thinks that his course is dictated by logic, but Cassius—who wants to kill Caesar because he is jealous—has used cunning to convince Brutus. Cassius suggests no direct actions at first, only drops hints, and even the notes he has Cinna throw into Brutus's window contain strategic blank spaces. Though it is Cassius's plan from the beginning, Brutus becomes the first character to explicitly state that Caesar must be killed. Though Brutus is probably the most intelligent character in the play, he is better at using this intelligence to govern his own actions than to control others; the speech he makes to the plebians after the murder is brief and spare, drawing only on logic. Antony, however—who combines the skills of Brutus and Cassius—turns the crowd around with a much more effective speech, involving both logic, emotion, and skillful speaking "tricks" such as visual aids, audience participation, and suspense.
All the major characters of Julius Caesar are public figures—some are even like celebrities—and are conscious of the fact that they live their lives and make their decisions before the audience of the Roman people, who may or may not be receptive. They are also careful about the personae they project in front of one another. Caesar is careful to always present himself as fearless and steadfast, even in front of trusted friends like Antony, and walks half-knowingly into his murder because death would not be as bad for his image as making an effort to avoid death. Though privately he is ailing and superstitious, Caesar would not be Caesar if he did not make himself out to be invincible. Cassius makes a show of being honorable, but is privately hypocritical and corrupt. Even Antony, who appears to be a "man of the people" and a loyal friend, plans to cheat the people out of Caesar's legacy, and to betray his partner Lepidus. And Brutus, who would otherwise be straightforward and consistent throughout the play, pretends in front of his troops to be unaffected by his wife's suicide.
Since the Rome of the play is the pinnacle of civilization, arguments about how it should be run are also arguments about what constitutes an ideal government. The entire play centers around Brutus accepting the truth of two moral statements: First, that Rome must not become a monarchy; and secondly, that killing an as-yet-innocent man is morally acceptable if it prevents Rome from becoming a monarchy. Brutus's strict moral code makes no allowance for self-preservation, however, and so he flatly rejects Cassius's suggestion that they also kill Antony, and even allows Antony to address the plebians. Giving in to Cassius on either of these points would have prevented Brutus's ruin, but violated his principles.
The attitude Julius Caesar takes towards free will is paradoxical. On the one hand, the human capacity for reason plays a chief role, as many scenes involve characters going through careful decision-making processes or engaging in complex arguments—this suggests a world where events come about as a result of human free will. On the other, many of the play's key events are successfully predicted, both by humans with prophetic abilities, and by the natural world itself, which makes signs out of weather, animal behavior, and even the reversal of life and death—this suggests a world where fate is predetermined, or at least heavily influenced by an unseen force, possibly the Gods.