Your characters, whether they're human or alien, act out of
who they are. They act out of their particular needs, cares, and values—and out of their flaws, as well.
In a sense, all plot depends on the discovery of character. Your characters have their own problems and values (which the author personally might or might not share), and in the face of the tensions and conflict in their lives, your characters must make choices, just as we do in real life. The conflict may occur between two or more characters, or between characters and the larger world around them, or between characters and their own inner voices. Regardless, the events of the story will follow from the choices made by your characters.
The sword cuts both ways, though. You may learn, in the telling, things you didn't know about your own characters. Sometimes you have to see them under fire before you know what they're made of. And you have to be prepared to see them grow and change over the course of the story.