"Caesar, beware of Brutus...defend thee"(2:3:1-8)
Act III: Sc
Speak hands for me! (3:1:78)
The soothsayer saying this to Caesar plays a big role in the play because it forshadow's what's going to happen at the ides of march. If Caesar would have listened to the soothsayer the whole play would have been different. The play probably wouldn't have ended with Caesar dying in the third act. Or Antony wouldn't have declared a war.
Act IV: Sc
"To tell thou shalt see me at Philippi" (4:3:283)
For the second time someone is trying to warn Caesar about Brutus and he does not listen. If Caesar would have listened to different people then he might have been alive. Even if he would have listened to Artemidorus and not the soothsayer the story would have been different. Caesar could have fought everyone back when they tried to stab him. Or could have had an army ready for him.
Act V: Sc
Caesar dying was a really big thing. It was the main reason for everything that happened after. The reason why Antony declared a war or convinced the whole town to hate Brutus and Cassius. Also another reason why they left town and had to get an army was because everyone was mad they they killed Caesar.
"Et tu, Brute"(3:1:79)
This foreshadow's another upcoming event. Brutus and Cassius will die at Philippi and Caesar is telling him he will be the reason for it. Also tells the audience that something bad is going to happen.
"Even with the sword that killed thee."(5:3:46). Brutus and Cassius both killed themselves and blamed it on Caesar. They both knew that they were going to die either way. Either they would become prisoners or slaves. Brutus promised that he would kill himself over having to walk through rome and he did.
I thing the theme is don"t trust people so much. The people Caesar thought were his best friends turned on him and killed him. Also Brutus trusted the Romen people so much and they as well turned on him. If Caesar would have taken some precaution with his so called "friends" he might not have ended up dead.