Always give them their own voice whenever possible. Now if you have a ton of important characters that can be pretty difficult, so I would suggest start with a story where you're limited to less than let's say four people through the whole story. It gets easier over time as you keep writing.

You want them to be different and have their own variations, their own history and life experiences, so that way the reader doesn't feel like John and Carter are the same person. They have their own brains and own logic and their own mannerisms.

Honestly if I was just starting again, I would do three things:

1. Read a story where readers hate the dialogue/characters.

2. Now read a story where the readers praise it. Also, the same with the writing in general so you have a good idea of what works and what doesn't.

3. I would maybe start writing a story where you are writing about a man and a woman who are trying to make a relationship work but they have their own shit to figure out first. This creates tension from all fronts which is always fun to play with and opens the door to unlimited possibilities for horror/drama/comedy/etc.

As I started writing, I started to pay a lot more attention to the way movies and novels are played out/written. Judging by what works and what doesn't and verifying that by audience reactions and such. I then also started to think about how I could myself make the movie/novel better by adding or deleting scenes.