I need to write a letter to a character in the novel trying to persuade him/her to change a decision that was made.

As i have struggled throughout this novel and rush to get my novel study done what is a good decision i could use in my letter to persuade the character not to do. I also have to explain what would be different if a different dicision was to be madeproviding details that have insight on the characters thoughts and personality. Giving details from the novel aswell. Thanks in advance