Every novel that has ever been written involves characters interacting with one another. Even when a novel's storyline is around loneliness, you may find a character has a relationship with an inanimate object or animal. Without character interaction, the story cannot move forward, and there can be no conflict. Without conflict, there is a little chance of good character development. When you have created realistic, three-dimensional characters, then the most interesting aspect of the story can be how they connect with each other. A character that is confident in a business meeting may go to pieces when their secret crush is around. A shy introvert housewife may turn into a vicious fighter when the safety of her children is put at risk by her violent partner.

There are four essential components that any relationship can have. If you are composing a manuscript around a relationship, for instance, you will need to establish the relationship foundation first, and then create the character qualities to fit around that relationship.  

The characters have a typical bond that both unites and keeps them together. This is most generally found in crime and thriller novels. For example, there are two policemen, both after the same villain, but they have differing views on the best way to capture him. They have a common goal and that is what keeps them together and bonds them in a relationship, even though they dislike one another.  

There must be a conflict between the characters for the story to progress. There is no such thing as a perfect relationship, whether in novels or in real life. Just as a common goal keeps a relationship together, then there must be a conflict to threaten the characters' relationship also. This can manifest itself by showing a difference of opinion towards an event, because they have different beliefs.

If you have created a good, strong backstory for your main characters, then you can introduce conflict between them very easily. One character could play by the rules, whilst another is always trying to challenge their viewpoint and get the matter resolved by the quickest method possible.

The relationship will change both characters by the end of the novel. For better or worse, there has to be a change in their outlook towards each other. Maybe the straight character will loosen up and be prepared to break a few rules in life, while the underhand character will find a heart and compassion.

If you include these four elements in your story, then it will keep the momentum and the readers' attention as they follow the constant conflict between the two characters. Try to remember the friendships you have had in your past. How different were you when you both met? Did you have differing opinions or personalities? How has this changed as the relationship progressed? Do you have any common values now? If you try this exercise for a few of your relationships, you will recognise a pattern emerging and that can be the blueprint for your character conflict.