Writing help

Relationship development for Characters

Relationship development is important, not just between your main character and those they love, but also those they hate. Make sure your antagonists (the ones that are people) have developed relationships with your main character as well. It will add dimension to both your conflict and your antagonist.
At its core, relationship development is based on the same principles no matter the relationship, whether the characters in question are sworn enemies, best friends, or lovers.

1. Develop your side characters
It's hard to have really well developed relationships if the only character that is developed in your main character. While you might not put a lot of the development of your side characters in the book itself, you should know everything about them.

2. Start out slow
As much as you want your characters to fall in love, be friends, or loathe each other immediately, it's just not realistic in most scenarios.
For an example I know everyone is probably familiar with, in Harry Potter, Draco and Harry didn't immediately hate each other. Draco made overtures of friendship when they met, and then again later. Their antagonism built with every confrontation they had.
Take your characters' personalities and the situation of where they meet into account when deciding how they will start off their relationship. It's fine to have your characters "click" but make sure your readers can connect why your characters got along so well. For example, using Harry Potter again, Ron and Harry fall into a friendship, but it feels natural because of how they met and their respective situations.

3. Build their relationships consciously
Be mindful of the pacing of your relationship development, and make sure it feels natural. Some ways your characters can get closer (or further apart):
Shared experiences
Communication
Shared interests
Time spent together
Each interaction should strengthen or weaken their relationship. No matter what device you use to build their relationship, make sure it is natural to your characters' personalities. Someone who has issues with trust is not going to spill all of their secrets in a bonding moment unless significant trust has been built up.

4. Show the development
Highlight the development over the course of your story by showing characters getting closer. What form this takes really depends on your characters, but they could share secrets more easily or be more likely to show physical affection with hugs, or simply spend more time together. Each scene that they interact should show their progress.
However you choose to show it, it will be more prominent if there is a direct contrast between early in their relationship and after there has been significant development.

Specific Relationships

Friends
Make sure your characters are friends outside of plot requirements by treating their relationship with respect. That is, your main character should not just use their friends as pawns/weapons (unless that is one of your main character's traits), and their friendships should not be completely overshadowed by any romantic relationships.

Lovers
Make sure attraction is not the sole connection between your characters. It can be in the beginning, but for a lasting romantic relationship to develop, you need to have a more substantial foundation. Build up the trust between your characters and make sure they communicate if you want to portray a healthy relationship.