Many books on writing will tell you that a good story requires character conflict. In fact, this is far too limiting. Just as with real people, characters can relate in ways other than by coming into conflict which are just as strong dramatically.
Though conflict is an essential part of a story, there are three other kinds of relationships that are just as important:
1. Dynamic relationships foster conflict. Positive Dynamic relationships are like the "loyal opposition" where two sides butt heads, but synthesize a better solution because of the conflict. Negative Dynamic relationships occur when two sides butt heads until each is beaten into the ground.
2. Companion relationships involve the indirect impact one character has on another. Positive Companion relationships occur when there is beneficial "fall-out" or "spill-over" between the two sides. For example, a father might work at a factory where he can bring home scrap balsa wood that his son uses for making models. Negative companion relationships involve negative spill-over such as a room-mate who snores.
3. Dependent relationships describe the joint impact of the two sides. For example, positive Dependent relationships might bring Brain and Braun together so that they are stronger than the sum of their parts. A negative Dependent relationship might have a character saying, "I’m nothing without my other half."
4. Associative deals with the relationship of the individual to the group. Rather than being consistently positive or negative, the two varieties of this kind of relationship may be either - but in any given relationship one variety will be positive and the other negative. The Component variety sees characters as individuals. The Collective variety sees them as a group.
For example, two brothers might fight between themselves (Component), yet come to each others’ aid when threatened by a bully because they now see themselves as family (Collective).
If you limit yourself to exploring only the conflicting relationships, ¾ of the ways in which people actually relate will not appear in your characters. What’s worse, if you also limit yourself to using only negative conflict, 7/8 of real relationships will be missing in your story.
By exploring all four kinds of relationships in both positive and negative modes, your characters will interact in a full, rich, and realistic manner.
Keep in mind: believable characters are not only built by developing each independently, but also by how they relate one to another!