How has Beatrice's and Benedict's language changed throughout Shakespeare's Much Ado About Nothing ?
In the opening scene when we first meet Beatrice and Benedick, we see that Beatrice uses a lot of rhetorical schemes to insult Benedick, while Benedick makes more use of figurative language. By the time they have fallen in love and confess to each about their love in the fourth act, we see that their language, especially Benedick's, has become more straight forward although Beatrice does still occasionally cling to her use of rhetorical schemes.
We see Beatrice making use of the rhetorical scheme epistrophe in order to sarcastically insult Benedick. Epistrophe is a rhetorical scheme in which the ending words of sentences are repeated (Dr. Wheeler, "Schemes"). We see epistrophe in Beatrice's questions to the messenger when she asks him how many men Benedick has killed in the war, as we see in the lines:
I pray you, how many hath he killed and eaten in these wars? But how many hath he killed? For, indeed I promised to eat all of his killing. (I.i.35-37)
Because the word killed is repeated at the end of both of these rhetorical questions, this is a good example epistrophe. In this passage, Beatrice is sarcastically criticizing Benedick's ability to fight by saying that she "promised to eat all" of the men he killed, meaning that she believes Benedick killed zero men in the war he has just returned from.
While Beatrice sometimes also uses figurative language to sarcastically portray Benedick, we see Benedick using more figurative language in his lines addressing Beatrice. For example, Benedick chastises Beatrice's ability to insult others by calling her a "rare parrot-teacher," meaning that she could teach a parrot to only speak insults, like herself (I.i.118).
However, after they realize that they are in love with each other, their language becomes much more straight forward and direct, especially Benedick's. In Act 4, Scene 1, in which they confess to each other that they love each other, we see examples of Benedick's straightforward language when he assures Beatrice that he believes Hero is innocent, saying, "Surely I do believe your fair cousin is wronged," and also when he confesses his love for Beatrice, saying, "I do love nothing in the world so well as you" (IV.i.270, 277-278).
However, while Beatrice does respond with some straightforward language of her own, especially when she asks Benedick to kill Claudio, as we see in the line, "Kill Claudio" (297), she also resorts back to using rhetorical schemes, especially when she confesses that she reciprocates Benedick's love. Beatrice makes use of the rhetorical scheme antithesis when we see her tell Benedick not to believe her confession, as we see in the lines, "But believe me not; and yet I lie not. I confess nothing, nor I deny nothing "(280-282). An antithesis is a sentence that portrays two contrasting ideas (Dr. Wheeler, "Schemes"). In this passage, the clause saying "I lie not" contrasts with the idea of telling him not to believe her. Likewise, the ideas "confess" and "deny" are also opposites, showing us that these pair of sentences are perfect antitheses.
Hence, Beatrice's and Benedick's language changes from being full of rhetorical schemes and figurative language to being more direct; however, Beatrice still holds on to her use of rhetorical schemes.