[Gallagher]

Right, people underestimate the exaggerations shown in things like action movies. A little research on things like basic anatomy can go a long way, even if you think you know things because it's your own species or language or you 'took a class once'. No. Do your research and actually remember it because it matters.

Always do time period checks. Always make sure you have a grasp of the setting and the rules for tech/magic. They differ between different universes. A lot.

It's easy to say you have a character with elemental powers, but think in depth. What are the limits? How would it work in _____ type of universe? Would it work in that universe?

Good characters aren't good because they're super-awesome-amazing. They're good because love went into making them believable in the universe they're playing in.

Get the biology right. It's important. Figure out where they come from, what the social and political systems were like, how the world worked around them. Figure out their history, their family, their friends — not just that they had a mother, what kind was she, free or strict or religious or distant — and what happened to them. Traumatic history? Research PTSD and other issues the character might have suffered, or is still suffering. If you want a good, believable character, it all matters. Even things you think you already knew.