How would one describe characters in Shakespeare's Romeo and Juliet, such as Romeo, Juliet, Mercutio, etc?
By character description, it looks like you are aiming to write what we would consider to be a very brief character analysis. There are many different things we can consider and analyze for when writing character analyses, and many ideas to analyze for can be found in the article "How to Write a Character Analysis." Due to space limitations on this website, we'll go over some things to look for when analyzing your characters plus one example.
When doing a character analysis, we are basically aiming to figure out exactly how the author is portraying the character as a person. Some important things to think about is how the character speaks, what he/she says, and what he/she does. It's also very important to look at how other characters respond to the character in question. We can also consider whether or not we think the character makes ethical choices and what the character's driving motives are, as well as many other things.
Looking at Romeo as our example, we actually learn a great deal about him in the very first scene. Think about what we learn of his emotional state in this scene and why it is significant. His father is worried because Romeo has been seen every morning at dawn, crying under a grove of trees in a certain part of town. We then learn that Romeo is feeling heartbroken due to Rosanline's rejection. But is this true love Romeo is feeling, or just infatuation? Plus, more importantly, what does Romeo's persistence in continuing to pine over Rosaline say about his character? We learn that his feelings for Rosaline are really more infatuation or lust when he explains to Benvolio that she has made a vow to remain chaste, just like the goddess Diane, and refuses to be seduced by Romeo, as we see in Romeo's lines:
She hath Dian's wit,
And, in strong proof of chastity well arm'd,
From Love's weak childish bow she lives unharm'd. (I.i.211-13)
Yet despite the fact that his feelings are really more infatuation or lust, he continues to mistake real love for lust, just like most young men. Hence, from this one scene alone, we learn that Romeo can be described as young, emotionally driven, and lusty. The moment when Romeo also refuses to listen to Benvolio's advice to forget about Rosaline is also very important. From this, we learn that not only is Romeo young and emotionally driven, he is also irrational, and these are character traits that not only define his character throughout the play but also move the plot forward.