This is a writing exercise to help develop your supporting characters. Start with a scene from your latest work in progress. The scene should contain a fair amount of dialog between your main character and one other character. The object of the exercise is to rewrite the scene so that the dialog is between your main character and a different supporting character. The goal is to determine how each character would respond to the main character in the scene. For example, if your scene is a confrontation between the MC and the school bully (antagonist,) where the bully confronts the MC in the parking lot at school and tries to pick a fight over a perceived slight, change the scene so that the confrontation is between the MC and his mother over a perceived slight. (Leave the setting the same.) Then change it to the MC and his best friend over a perceived slight. What has to change? How do you maintain the tension of the scene? How does the atmosphere of the parking lot change? And, do the voices of each of the characters ring true?
IF you don't have a WIP, try writing the scene described above, between (1) MC and a bully; (2) MC and his mother or father; (3) MC and his best friend; (4) MC and his girlfriend.