Characters start within you, the writer, and have a connection to your heart whether you recognize it or not. If you try to write a truly brutal person, but your heart doesn’t have that kind of aspect to it, then you’ll end up with a cardboard character. This is your first step.
The second step is that personality comes from actions, thoughts and speech patterns. How you chose to have them talk is a great way to show difference of personality. Do they all speak the same? Are their patterns identical? Can they make someone laugh or cry? How about the way they think? Does each character have identical thoughts and values? If they do, then you need to change that. Just as in real life, each person acts different, but it all your characters behave the same, then they’re cardboard characters.
As a whole, characterization isn’t the most difficult part of writing. Making sure your story follows a logical path, doesn’t have as many hole as Swiss cheese, will prove to be the hardest part.
Hope this will help those with questions.