The short answer is practice.
Try writing short stories. Take a scenario, work on it. Read it. If you are satisfied, as in you have good grasp of the emotions of you character, and have learned to put them down on text, you are good.
The Long Answer is ... some literary theory? Most writing does not try to follow life-like realism; not even the great Realists, such as Chekhov or Dostoevsky, try to present you a truly well rounded character. Instead, the character is nothing but an instrument for the main conflict of the story to get resolved.
As an example, Rodion Raskolnikov from Doestoevky's Crime and Punishment does not in fact represent a complete picture of either a person suffering from mental illness or a person who has committed a murder. He fluctuates between them, morphing the story as he goes. The story reacts to his thoughts, and not vice-versa - which is how we humans live. We react to our surroundings, whereas Raskolnikov's story moves through arcs.
So, being life-like is a detriment. In post-modern novels, this is even more apparent. Waiting for Godot is based in absurdity. In modern Young Adult fiction, the characters are simpler. They are not showing philosophies but usually focus on extraordinary tales. E.g. A Fault in Our Stars.
To be more quantifiable, if a life-like character needs 25 Dimensions, show only the 3 that are changing in the story. Focus on that. You cannot write something like Ulysses where you would go into the consciousness of characters. Make the story react to your characters. Understand why two characters will connect, and why two will disagree and fight.
But remember that each decision is weighed by many different external forces that affect the character - such as their parents, love, desire, hopes, fears and their experiences, or lack thereof.
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I think my answer is all over the place to be honest.