Characters who seem like flesh and blood people to readers is an essential part of good fiction. Mainly this requires that a character is multidimensional - just like real people. A character needs to have a past, hobbies, issues, dreams etc.
One way to get to know who your character is in-depth, is to imagine him or her in all sorts of scenarios. It can be especially helpful to include situations not related to your story at all. In my novel, "Love, Sex, and Understanding the Universe," my main character, Jim, never once goes to a beach. But when I was on vacation during the time I was writing the book, even though I did not write during the vacation, I took the opportunity to - in my head - imagine Jim on the beach in Mexico. What circumstances would have brought him there? Who would he be with? What would they be saying to each other? What would he like or dislike about that beach? What kind of swimming attire would he wear? What beach activities would he partake in? Would he use sun block? etc.
Once you have a real grasp on the people you people your novel with, it will come more automatically to you to know how they will respond to the situations in your book, what they will say and how they will say it.
One of the things the other writers in my writers group use to say to me when I read from "Love, Sex, and Understanding the Universe" is "you really get inside Jim's head!" The truth was, I told them, that Jim really got inside my head.