Through this scene Shakespeare reveals the pressures and the nervous tension and anxiety in the maritial relationship of Brutus and Portia.
Brutus has spent a restless and sleepless night and convinced himself that it is best to murder Caesar for the common good of Rome. He has discussed with the other conspirators all the finer details of the assasination that has to be carried out the next day. Brutus' final word of advice to the conspirators is: "Good gentlemen look fresh and merrily/Let not our looks put on our purposes."
Portia enters immediately after the conspirators had left. It goes without saying that Brutus cannot reveal to his dear wife why he has not slept the whole night and who the late night visitors were and what he discussed with them, even though she pleads with him on bended knee. Brutus offers evasive replies, and fortunately for him Ligarius, whom he had sent for, is heard knocking at his door. Brutus hurrriedly asks Portia to leave saying that he will reveal everything to her later, "and by and by thy bosom shall partake/The secrets of my heart...Leave me with haste." And she being the obedient wife does so.