Make sure your main characters are prominent and well fleshed out.
Don’t be tempted to introduce too many main protagonists, or add them as you go along. Use your outline to help you consider which characters you will need and keep it to a minimum. If there are too many, you risk upstaging your protagonist. Again, this can be fixed if you have already started, or completed your first draft. Go through it chapter by chapter and consider whether their action is in character. Can it be
To be believable, characters must be multi-faceted and nuanced. Make their wants and needs clear. Even in fantasy, nobody is either all good, or all bad. ‘Good’ characters are not flawless. What makes them interesting is how they overcome those flaws. Nor are ‘bad’ character’s all bad. Everyone has reasons for acting the way they do and these must be made clear to your audience. However, these do not have to be revealed all in one go. For instance, the Evil Queen in the TV series Once Upon a Time, acted out of revenge against Snow White whom she felt was the party responsible for the death of the young man she actually loved. She was then manipulated, by her power-hungry mother, into a loveless marriage to a man three times her age just because he was a king. The Evil Queen became consumed by her grief and went on a murderous and devastating campaign to revenge herself against the cause of that pain. The series opened with the promise ‘to destroy the happiness’ of everyone in attendance of the wedding of Snow and Prince Charming, and worked back, revealing Regina’s path to that moment.
In Orson Scott Card’s ‘Speaker for the Dead’, (if you have not read this, I recommend you do) the now adult Andrew Wiggin having discovered the enormity of what he had done as a child, had never returned to earth. His own fictional work ‘The Hive Queen’ and the ‘Hegemon’ had reduced the name ‘Ender’ from a symbol of heroism and saviour of humanity, to an epithet ‘Ender the xenocide’. These books told the whole truth of the situations of the people about which they were written with nothing hidden. The Speakers were not there to give either vilifying post-mortem expose’s or shining accolades, but to give a true accounting of the life of the dead with all their flaws and virtues laid out. This is how to treat your characterisation.
Individual character bios could help you to get to know them and an even better perk, is that they can be updated for use in sequels or series. Don’t go into too much depth with these though, especially with characters’ physical appearance. You don’t want to end up bogged down in small details, but it will enable you to refer back to earlier books if you have crib to work from. Index cards are a very easy way to do this. If you are feeling especially adventurous, a character database would also be useful.