Beatrice closes Act 3 scene 1 of Much Ado About Nothing, speaking a sonnet.* Shakespeare occasionally used sonnets in his plays, for example, in Romeo and Juliet and Richard III, which were examined in previous essays. He didn't insert these sonnets arbitrarily. He intended to achieve effects, which is clearly the case with Beatrice's sonnet.
And Benedick, love on; I will requite thee
What makes Beatrice's sonnet effective is not its quality. It's certainly not one of Shakespeare's best pieces. Her sonnet is effective because it's the first time in the play that Beatrice speaks in any verse at all, let alone in a sonnet. That makes this moment striking, especially to Elizabethan audiences' ears. It also provides the actor who plays Beatrice an opportunity to rise. The effect would be similar to a character in a musical breaking into song after speaking only prose dialogue for the first half of the play. Beatrice, having spoken in prose and, to various degrees, having dominated her previous scenes, suddenly turns lyrical, suddenly seems vulnerable. And this marks a climactic turning point in a main character midway through the play.
However, she still speaks her sonnet in character. The sonnet's lines are almost prose-like in their simple declarative sentence structure and, while they are not lacking in rhetorical devices — “What fire is in mine ears?” “Taming my wild heart to thy loving hand” — Beatrice speaks them in her recognisable voice with her characteristic spunk — “And Benedick, love on; I will requite thee.“
Too often Shakespeare is regarded as a demigod of literature, divinely inspired by muses that ordinary writers will never hear. And while it is true that he is the greatest writer of the English language, maybe any language, he employed ordinary tricks that ordinary writers might easily emulate. Beatrice's sonnet is one more example of writing that is within any author's reach.
* Beatrice speaks an abbreviated sonnet — two quatrains and a couplet rather than three quatrains and a couplet. This is similar to the last sonnet in Romeo and Juliet, which is only one quatrain and a couplet. Shakespeare sometimes tweaked the sonnet form. For example, he sometimes wrote sonnets in iambic tetrameter (four metric feet) rather than pentameter (five), or in fourteen lines of seven rhyming couplets. This type of experimentation with traditional forms is characteristic of many Renaissance artists. And as Shakespeare matured, he experimented even with his plot structure and genre form. See Plays by Year and Genre.