What events or scenes in Act 2 contribute to the suspense? Why?

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.