Dogberry, Verges, the night watch, and the Sexton clumsily interrogate Conrade and Borachio. Instead of focusing on the important matter in the case—that Claudio and Don Pedro have been tricked into doubting Hero’s faithfulness—Dogberry becomes obsessed with minor matters. Throughout the interrogation, Dogberry comically misuses language: meaning to say that Borachio has called Don John a villain (which he does not think is true), he instructs the constable to “Write down Prince John a villain.” (4.2.42) When Conrade insults him, he asks “Dost thou not suspect my place?” (4.2.74) meaning to say "respect" instead of "suspect." The Sexton and members of the watch get around to finishing the interrogation and reveal the prisoner's guilt in faking Hero's lack of faithfulness. Dogberry becomes offended when Conrade calls him an ass, repeats the insult several times, and imagines how horrible it would be if that comment had been written down. Finally, they all head to Leonato’s to deliver their report.
“Comic relief,” characters like Dogberry are usually placed by Shakespeare in his tragedies, to relieve tension. Much Ado is already a comedy, but Dogberry makes his appearance just as the play seems about to take a tragic turn. Dogberry’s misuse of similarly-sounding words is there for its comedy. But his “accidents,” often convey something truer than what he means for them to say: Don John is in fact a villain, and Dogberry’s incompetence does make his position suspect. Dogberry is so offended at being called an ass that he repeats it for the rest of the play, making himself look like an ass.