In my Hernandez series, we can see the relationships between characters and how they evolve or change throughout the story. For example, the protagonist, Jorge Hernandez, has a powerful effect on others. He brings out everything from fear to admiration, and loyalty, depending on the character he's dealing with and the specific situation. We can sense the strength in his personality and how people react in turn. For example, some characters try to impress him, while someone like his teenage daughter instead might challenge him. His reaction to each also tells us a little more about who he is and his values. That's the beauty of character interaction; it allows us to see what pushes their buttons and what brings out both their best and worst traits.
One thing to keep in mind is that these connections change over time. Characters grow, some fade from the story, and others devolve. This is much like the people in our lives. It's interesting to see how each plays off the other, creating fascinating plot twists and subplots.
The most important thing is to bring out the 'human' in characters, and really, is there anything more human than our relationships and interreactions with one another?