If I were Mercutio (before he was killed), I would be angry with Romeo for spending all his time moping over a girl when he should be out having fun with the boys. I would want to persuade Romeo to get over Rosaline and go back to being happy. I'd probably try to tell him that his obsession wasn't going to get him lucky because he'd already tried everything he could think of--he's given her the eye, talked of love and even tried bribing her with "saint seducing gold."
Mercutio probably feels abandoned by Romeo. First with Rosaline, and then again with Juliet. He is also mad at Romeo for being too much a lover and not enough of a fighter. He said he thought Romeo would have answered Tybalt's letter (which was a challenge to fight) but of course Romeo doesn't do this, which makes Mercutio angry. You can't go too far beyond that, though, because shortly after Mercutio jumps in to defend Romeo's honour, he is killed. So unless the ghost of Mercutio is writing the diary, you'd have to cut it off.
What is the topic? You might want to have him write prior to his death...maybe given some foreshadowing to hint at his demise.
April 23, 1498
My idiot friend Romeo is still pining for Rosaline. What an idiot! I wish he was more like me. I'm free spirited, young, and can take care of myself. He should meet this cool chick named Juliet. I hope he meets her at some party, but he spends too much time over girls. I have to teach him fencing so he can take care of himself. He is getting to feminine and I can't have that.
That is the beginning of his diary...You should be able to finish it.
well there is this assignment about mercutio that i have to do a 4 page diary about him.i have to include persuasive with in this case ur mercutio and you have to persuavie someone .any advice on who to persuade and why.