Ways to build an interesting character

1. Give them a purpose. Characters that have something to work toward have a reason for everything they do, and for those things they don't do.
2. Have them act. A character who sits around doing nothing can quickly become boring. A character who is actively working throughout the scene more easily keeps your attention.
3. Let them think. When the moment is right, let the character think about their situation. Don't monologue (unless the character is a super villain), but have their thoughts bring the reader a greater understanding of the character.
4. Get them in trouble. How does your character react to trouble? Are they the perfect student who is horrified at being sent to the Principal's office? Or are they the one who charges recklessly at the werewolf so that others can escape?
5. Let them be afraid. What a character fears shows us a part of who they are. How they handle that fear shows us what they can become.
6. Show us what they love. If the character loves something, show it to us. If we see the character's care for that object (or person) then we may grow fond of it as well, and stick with the character when that love is endangered.
7. Don't hide their flaws. A perfect character is dull. A character who makes mistakes, suffers setbacks, and has their faults brought out to the open is a character who has the potential to grow. Let them rectify those mistakes, learn from those obstacles, and face their own weaknesses. And when they've defeated those flaws, reveal more.
8. Let them interact with others. How does the character change when others are around? What role do they slip into when holding a conversation? The way the character acts around others can reveal a lot about themselves.
9. Give them time alone. During those quite moments, a new side of the character is revealed. What do they do when no one is watching? What masks are removed? When the character is alone, do they long for companionship? Or do they rest, lowering their guard just enough that the fragile creature within is exposed?
10. Let them have fun. There are times when the character needs a moment of laughter, a break from the trials they've faced. Let them have that moment, that chance to remind us of the person they were at the beginning of this journey.