One of my favorite techniques for building a fictional character is also the most fun. Before I tell you what is it, I'll tell you how I learned it.
I wrote my first three books by starting with a good premise. It seemed enough, and although I'd heard of using an outline or storyboard, I wondered if they were too limiting.
But after I wrote myself into more than one corner in book three, I sought out "Prescription for Plotting" by Carolyn Greene, a sister member of Virginia Romance Writers. Her kit included worksheets to help you develop your plot, characters, events and turning points.
The take-away that has stayed with me is definitely Carolyn's tools for fleshing out your characters. You get a worksheet with boxes to fill in which describe your character's attributes, physical features, personality traits, favorite foods, pet, birth place, flaws, etc. I hadn't considered what "car" my hero would drive, especially when he was a Viking. But, ah! I researched more about his ship and it became a key element in my story. I learned that his favorite color was blue. (Same as most men.) And he loves to drink mead.
Like most research you'll do, you'll only use some of it. The one element I do include is that fun technique I mentioned: a character tic.
Go online and you'll find lists of character tic examples, but don't just have your heroine twirl her hair, tap her foot, or use a cute expletive. (And while I'm on the subject, please NEVER have your character bite her lip or chew on her lip. It's laughably overused.)
Instead, have your character's tic come from an event in their past, their profession, a deep need, or emotional wound. For instance:
She limps a bit on rainy days from the car accident when she was a child.
He jumps when he hears a distant siren.
She reaches for her stethoscope even when she's not wearing it.
He finger combs his hair, worried that the scar might show.
She pulls the hood of her cape low.
What are your favorite techniques for building your characters?