Most of my characters aren't aliens.
Many of my stories begin by imagining the character first. The world and everything else is then built around this person ... or alien. I usually see the opening scene, much like a movie, in my head. Then I have to figure out how to invent a whole novel or story around that character and scene.
I usually do some rough drafting before nailing down details. It's through that initial frenzy I learn more about my character. I can't tell him or her, she or he will tell me as the story and world unfold.
At that point, I'm ready to delve more deeply into a character. There are many ways to do this, and it's as individual as a fingerprint. I like to write sketches in a journal -- a brief bio of my character's life before the story begins, my character's foibles and habits, and I sketch out their arc -- how they change over the course of the story. I usually find a photo or piece of art to depict that character. Usually there's something in the piece of art that screams some trait I want to develop.
To get deeper into character, I decorate their personal spaces, fill their closets and jot down a lexicon so that I can paint the world through the character's senses. Every one of us has a unique lexicon. Writers have one. Every profession and region has one.
I also usually create a part for myself in the story, so I experience it beside my character. I tell myself the story over and over, seeing it like a movie. I tell myself the story every night before I go to sleep. It actually helps me fall asleep. At any rate, I go to sleep thinking of the character and story. When I wake up, I get to writing.
How do you get to know your characters?