Don John complains to his companion Conrade (Conrad in some editions) about his position in life: He is Don Pedro's bastard brother, recently defeated, without pretenses or mannerly habits, facing his dishonorable status daily while enduring his brother's hospitality. All in all, he displays a generally disagreeable attitude and seems determined to make the most of it. His second companion, Borachio, enters to report having overheard (noted) the conversation between Don Pedro and Claudio wherein "the Prince should woo Hero for himself and, having obtained her, give her to Count Claudio."
Don John immediately sees this plan as an opportunity to do mischief both to Claudio — honored for his actions against Don John — and to Don Pedro. Don John and his men head out for the celebratory supper, Don John expressing regret that the cook is not on his side and ready to dispatch the assembled household and guests with poison.
Although the audience was introduced to an apparently submissive Don John in Scene 1, in this scene in no way is he subdued. In fact, he feels compelled to create trouble for the two men who are responsible for his recent defeat — Don Pedro and Claudio. When Borachio reports having hidden behind a tapestry to overhear the conversation about Claudio's intentions toward Hero, Don John is cheered by the idea that he can use this situation as "food to my displeasure."
This scene includes the second reported incident of overhearing/noting. Borachio has eavesdropped on the same conversation that someone reported incorrectly to Antonio — but Borachio gets it right.
The character developed as Don John is sometimes criticized because he does not have any real motive for disrupting the marriage of Hero and Claudio. Rather, this scene provides a clear motive for getting back at Claudio:
That young start-up [upstart] hath all the glory of my overthrow. If I can cross him any way, I bless myself every way.
Because Don Pedro is supportive of Claudio, any action against Claudio will also be an insult to his brother — his ultimate target for trouble. The audience does not have to assume any innate or unexplainable streak of evil in Don John. The man does, however, seem to relish his villainy.