How does Popilius Lena cause tension and suspense at the beginning of the scene?

Caesar is headed to the Senate House with all of the conspirators surrounding him. He sees the soothsayer and tells the man that the ides of March have come. The soothsayer responds with, "Ay, Caesar, but not gone" (3.1.2). However, Caesar is not concerned and continues to the Senate. Next Artemidorus attempts to hand Caesar his letter, explaining its contents affect him personally, but Decius responds quickly, telling Caesar the Trebonius has a document for him to read instead. Caesar tells Artemidorus that, "What touches us ourself shall be last served" (3.1.7).