1. Add a gesture or the lack of one. Have a character that doesn't move when others might. Have a character that fidgets. Have a character that always coils hair around their fingers. Have a character that always nibbles their nails when they're thinking or bites their lips. Give them a gesture - YOU make them, so can they.
2. Know where they're going. I firmly believe you start a character at the end. If you want a character to end up a confident leader, have them start out with no confidence and always following. If you want a character to end up in love, have them start without it. If you want a character to end up with friends, have them start without any. Always start designing a character by what you want for them in the end, and doing the opposite.
3. Give them speech patterns. We have speech patterns, distinctive ones. So can characters. Internal thoughts, out loud, in first person - your character has a way of thinking, and a way of putting those thoughts out there. Have a character that never says more words than they need to. Have a character that thinks and thinks until they blurt things out and it never comes out quite right and then they have to explain it. Have a character that almost always just asks questions. Subtle but distinctive patterns of speech and thought can create stronger characters.
4. Clothing matters. What do they wear? Why? What personal objects do they carry with them? What is in their pockets, precious? Not just that, but colors, how they wear things, what they choose not to wear. How do they tie up their hair? What kind of shoes do they like? Do they wear a jacket, a cloak, a sweater? Are they one of those people who wears shorts in the snow?
5. Know how they grew up. Your characters were raised to believe certain things, raised a certain way. How did their upbringing affect how they present themselves? Toxic masculinity? Feminism? Soft and sensitive? Communicative? One of those people that always asks for what they want, or one of the people who tries to lead others around to what they want indirectly? Even just giving them the barest bones of an upbringing can help you form a character. Loving family, friends, no family, no friends, moving a lot, always living in one place - every aspect of those adds to a character's insecurities. If you have a character that always lived in the same place who then goes on an adventure, they will miss home. If you have a character that has always moved around who now has to stay still, they'll face entirely new challenges in maintaining social relationships.

“No writing is wasted. Did you know that sourdough from San Francisco is leavened partly by a bacteria called lactobacillus sanfrancisensis? It is native to the soil there, and does not do well elsewhere. But any kitchen can become an ecosystem. If you bake a lot, your kitchen will become a happy home to wild yeasts, and all your bread will taste better. Even a failed loaf is not wasted. Likewise, cheese makers wash the dairy floor with whey. Tomato gardeners compost with rotten tomatoes. No writing is wasted: the words you can’t put in your book can wash the floor, live in the soil, lurk around in the air. They will make the next words better.”