When it comes to developing characters, one of the things you might have to look at is how they interact with each other. A lot of interactions between people are based on previous interactions, which goes all the way back to when they first met.
Knowing how your characters met each other can help with more than just setting the tone of their relationship. They can also help you find hidden interpersonal conflicts and gives you a good base point of reference for how the relationship changes both in and out of the story as time goes by.
As an exercise: Set a timer for ten to fifteen minutes and free write the first meeting between:
- Protagonist and Antagonist
- Characters and Love Interests
- Protagonist and Supporting Characters
- Antagonist and Supporting Characters