Tybalt and Mercutio are stock characters—the fighter and the clown.
A stock character is a stereotype or a flat character. This is a character who does not change and has no depth, and could be in any story.
Tybalt is an example of a stock character. He is the hothead who causes trouble and cannot be reasoned with. Many stories have one. Tybalt sees Romeo at the ball and refuses to allow him to be there. He wants to fight Romeo even after Capulet insists that he has a good reputation. Later, he sees Romeo on the street and insists on fighting him again, even after Romeo does not want to fight. Of course Romeo does not want to fight, he is in love with Tybalt’s cousin Juliet! Yet Tybalt will not listen. Look at their interaction.
Romeo, the love I bear thee can afford
No better term than this: thou art a villain.(60)
Tybalt, the reason that I have to love thee
Doth much excuse the appertaining rage
To such a greeting. Villain am I none.
Therefore farewell. I see thou knowest me not. (Act 3, Scene 1)
However, it is Tybalt’s interaction with another stock character—the clown, that causes tragedy. Mercutio is Romeo’s friend. He also does not change. He is a punster and really more of a lover than a fighter, but he will not turn down a good brawl invitation. He is there when Tybalt accosts Romeo, and when Romeo intervenes between Tybalt and Mercutio, Mercutio gets hurt. This causes Mercutio’s death. Romeo then has to kill Tybalt—his lover’s cousin—in revenge. It’s all very messy and tragic, very tragic.
You are correct in noting that Prince Escalus and Friar Lawrence are also basically stock characters. Escalus is the authority in Verona. He is the one who decrees that the feud has gotten out of hand and the brawling must stop or anyone who takes part will be put to death. Then Romeo gets a lesser punishment, since he is young and of good character (and family), and Escalus banishes him instead.
Friar Lawrence is a little more complex. He is still not much of a round character though. He makes the potion that Juliet uses to fake her death. He sends the note to Romeo that never gets to him. He takes responsibility at the end, so you could say he does some growth. He’s your basic priest though-that’s what they do. Benvolio is your stock cousin-friend. Lady Capulet is a permissive mother, and Lord Capulet is an overbearing father. Both of them are pretty much standard and neither one changes. The Nurse, also, is a clown.
Is Romeo a stock character? He is your standard foolish lover in some ways. Does he grow? Maybe. Is he “round” ? I think you can argue that Romeo is complex enough to not be a stock character.
See how she leans her cheek upon her hand!
O that I were a glove upon that hand,
That I might touch that cheek! (Act 2, Scene 2)
He speaks poetry to his girl, he pines for one, then switches to the other, he goes out with his friends, he’s the typical teenager, but he does not fit any one mold. He certainly has passions. He kills Tybalt, and Paris. He marries Juliet in secret, he leaves the village and then comes back, and he poisons himself out of grief for her. Romeo goes through enough agony during the play to break the mold.