Dynamic characters are important and it's hard to write people you know nothing about. It'll show in your story if you try. I take pride in characterization being one of my writing strengths, and one way I do that is getting to know my characters as if they're real people.
I typically write in first person, so I want to get deep inside my narrator's head to make their narrative voice seem like a real person. To do that, I try things they like. I try to see the world through their eyes, at least for a few hours, to nail their voice.
For example, I was working with a character named Olivia for a long time. I was struggling with her voice and her character felt inconsistent to me. Olivia plays piano and I started listening to piano music while I wrote. Olivia also loves walking barefoot in the woods, so I tried it. By doing so, I felt like I understood how she sees the world better, and I learned that I also like barefoot walks in the woods.
For another example, I have a character named Juliet. Juliet is my favorite character to write, because she's... bright. Both her personality and literally. She only wears neon clothes. I dressed up as her for Halloween and I got to see what it was like to be her for a day. It was fascinating and I loved it. I own neon pink pants because of Juliet. And a neon dress. And socks. And two shirts.
Trying what they love helped me understand how Olivia and Juliet think. I began to notice that Olivia focuses on details, sounds, and textures. Juliet focuses visually. Olivia takes on the world with the gentle spirit of an observer. Juliet dives in with a loud, confident enthusiasm.
The more I got to know them, the realer they became to me. I'm not saying you need to know everything there is to know about your characters, but if they have something that makes them unique, try it. See the world through their eyes.
After all, in first person or close third, you are writing the world through their eyes. How can you do that if you don't know what it looks like? By knowing what Olivia and Juliet would notice, I was able to know what they'd focus on in a first person story. That gave them a sense of depth they hadn't had before.
Basically, the realer your characters are to you, the realer they are to the reader. The realer they are to the reader, the more the reader will care. The more the reader cares, the more they'll read.
Characters, not plot, are the best way to keep your readers hooked. Your plot can be fantastic, but no one will care if the characters in it don't sparkle with life. You want your readers to care, so you need to make the characters real. Get to know them. Try their quirks, their habits, their favorite foods. Listen to their favorite music, read their favorite books, watch their favorite movies. Get to know them the same way you'd get to know a person you desperately want to be friends with.
And who knows? You might learn something about yourself in the process. I sure did. Neon socks are barefoot walks are both things I love, and I never would have tried them if I hadn't gotten to know Olivia and Juliet.
Another way to get to know characters is by interviewing them, which I'll talk about in a few weeks.