To create more realistic interactions between characters(espescially main characters), it's important to remember that two seperate people will view the world differently. If that is the case, then seperate characters should also interact with the world differently.
For example, we have John, Rachel, and Fred. John is a casual, ordinary guy. He believes that you should go through life as easily as possible. Rachel is outgoing and hyper. She believes life should as interesting and exiting as you can make it. Fred however, is a more literal, precise kinda guy. He thinks a person should live realistically and practically.
See what I've got there? Three different people, three different personalities, and three seperate views of the world. But like what happens in real life, whenever you go out and interact with people, you still carry your views along with you into your interactions and conversations with others.
John, Rachel and Fred are meeting for lunch and discuss a vacation trip. One of the things Rachel insists on going mountain climbing. Fred believes they don't have enough money to buy equipment and should try a more financially practical vacation trip. John thinks they should just spend the vacation over at his house.
Here I've now added how they interact with each other. So in general, when you seek to make a well-rounded and interesting character with a unique view or perspective of the world, you must remember two things.
1. Separate characters should view the world differently. Whether a characters views differ religiously, personally, philosophically, or financially, each character should be distinct from the next.
2. Separate characters should then interact with their world differently. People carry their personalities with them into other activities and conversations. How a character interacts with other characters should be based on their personality.