Everybody has an agenda. We all have desires, hopes, and dreams. We all have principles. We all have goals, whether we formalize them or not. We all have a background and a historical perspective that shapes our actions and our outlook. In our interaction with others, we are at least somewhat aware that the person we are interacting has views and goals that may or may not be the same as ours.
Even the people we love, the people we support, and the people we usually agree with are individuals with their own way of thinking. Every interaction we have is colored by the perspectives and viewpoints of all people involved.
How often have you argued with somebody or watched two people argue when both sides are saying basically the same thing? That happens because we are all individuals and we each have our own agenda, and to some extent, we recognize that our agendas don't always agree, even when the points we are trying to make are the same.
So why should the characters in our stories be any different?
If you want your characters to ring true, they must each have their own world view, their own wants and needs, and their own goals. Their own agendas.
Characters on the same side take that position for their own reasons. Characters on opposite do the same thing. Your protagonist and antagonist might seem like enemies, and since your story is told from the POV of the protagonist (probably), the antagonist may seem evil. But from his point of view, he's probably taking his position as a matter of conscience, because he thinks it's the right thing to do. From the antagonist's point of view, and that of his followers, the protagonist is the bad guy.
But agendas are not limited to main characters. Every time a character appears in our story, even in the most minor of roles, we need to consider what that character wants. Maybe we don't need to create a detailed character analysis of our most minor characters, but we do need to know what each character hopes to achieve. Each character has a life outside the story, even if we don't know anything about it.
Too often, we write a character out of convenience, to fill a story need, without thinking about that character as a real person with hopes and dreams of her own. Usually, when we read and come across a character like that, we're unsatisfied. But still we write them.
Each person in your story world is there for a reason. Not just your reason, to fulfill a story need, but a reason of his or her own. Each character wants something out of his interaction with your other characters or your setting, or whatever he is there for. Even if the character is there solely to offer support to another character, he is offering support for his own, usually selfish, reasons. Even two characters who agree can have agendas that create conflict, and conflict creates story.
So remember that as you write. Every time a character is in a scene, consider why that character is there and what he or she hopes to get out of it. This is one of the most effective ways to turn characters into people.