Shakespeare spends Act II building up suspense about the murder of Julius Caesar, using dramatic irony to heighten the tension. We as an audience know from the beginning of Act II that Brutus and the conspirators plan to murder Caesar. However, Caesar himself and his wife and soothsayers don't know: the suspense hinges on whether he will properly interpret the clues that are warning him not to go to the Senate that day. (A similar build up of suspense occurs in Peter Pan. We as an audience know an attempt has been made to poison Tinker Bell but the suspense builds as we wait to see if she will actually drink the poison.)
Specific events that build the suspense:
Will Caesar pay attention to his soothsayers, who say he should stay home that day?
Will he pay attention to his wife's dream of a bloody statue of him and of people putting their hands in the blood and interpret it a warning of great danger?
Will Artemidorus, a friend of the conspirators who has found out about the plans, be able to get his warning letter to Caesar as he passes en route to the Senate?
An audience unfamiliar with the play (or the actual history) would wait with bated breath to see if the conspirators' plot was thwarted. As we know, it is not, but the suspense builds nevertheless.
There are tons of examples of suspense in this act.
1. Caesar had to be manipulated by the conspirators to go to the Capitol against Calpurnia's wishes.
2. The soothsayer, who has had an idea something would go wrong, is on his way to the Capitol to warn Caesar.
3. The augerer read the fortunes through a heartless animal, another sign Caesar ignored.
4. The teacher, Artemidorus, has learned some very specific details about the assassination attempt including all of the conspirators involved. (Remember that - teachers know everything!) He is on his way to the Capitol to also try and prevent the killing.
5. Portia is uneasy about what Brutus is involved in but still wishes to support him. Because of this, she sends Lucilius to the senate also to let Brutus know she is merry, that she is ok with whatever it is he is doing.
As a reader (or an audience member), we are aware that all of these plot lines are about to meet at the Capitol, but we don't know yet which will prevail. Of course, in the end it's Caesar's own arrogance that pushes aside the soothsayer and Artemidorus and eventually leads to his death.