In Atlas Shrugged explain how Eddie Willers' friend in the cafeteria & the shadowy man outside Dagny's office add to the novel's suspense.
The novel itself is not a mystery novel, or even a suspense novel; rather, it is a political allegory written to point out the dangers of a government that takes control of private sectors under the ruse of virtue. So, there isn't a lot of typical suspense or danger associated with that plotline. Adding a mysterious figure in the cafeteria, and outside of Dagny's office gives the novel and the rather detailed and complex political storyline, a bit of mystery and suspense. We start to wonder exactly who this person is, and how he is going to play into the entire story itself. When the figure is outside of her office, it even adds a bit of a threat to Dagny's life; all of a sudden, we wonder if she is in actual physical danger.
The man in the cafeteria is getting a lot of inside information through the innocent confessions of Eddie, and we wonder whether or not this person will use that information to harm the company or Dagny herself also. Not knowing who the character is adds mystery and suspense because we don't know if he is friendly or harmful; his intentions are unclear, and his motives and role in the story are not known, so we are left wondering and worrying. When we finally discover who he is, it is a rather pleasant surprise, one that makes things really come together in the novel. Until that moment though, we are left questioning and wondering at his identity, which is pretty suspenseful.
I hope that helped; good luck!