You need very little superficial details. Height, weight, hair colour, eye colour, race, identifying marks. The real importance in these details lies in what the character thinks of them. Maybe they think they’re too tall and tower over everyone, always stand out and can never blend in, or that they’re so short they’re always getting overlooked. They might hate everything about themselves or secretly love the way they look, but are worried about saying so because people might think they’re self centred. It is how the external image affects the character internally that fleshes out the character for the reader.
The only backstory you need to know about your character is what is relevant to them now. And even then, you don’t need to tell the reader right at the start. Nor should you provide too much backstory all at once and drag the reader from the action.
What Drives Your Character
What is your character’s motivation? What do they really want? And why. These are some of the main things you need to know about your character. These points and what is stopping your character from getting what they want is usually the driving force behind your story.
How Far Will They Go?
What is your character willing to do to get what they want? What aren’t they willing to do to get what they want? Where do they draw the line and what will push them past that line? And if you can get them to cross that line, how many other lines will they then be willing to cross?
Who Are They Now?
At the end of a book, the character may not be the same person they started out as. Is it a change for the better? Are they happy with this change? How do those around them now treat them? Have they reached their goal? Did reaching the goal give them the result that they were expecting? If not are they pleased with the result or dissatisfied?
I know it seems like there are a lot of questions, but when you meet someone for the first time you often ask numerous questions. You need to know your characters, particularly your main characters, better than you know yourself. You need to know how they’ll act or react in each situation you put them in. Likes, dislikes and outer appearance provide very little real depth for your characters. Their fears, morals and how far they’re willing to go to get what they want is what really lets a reader know your characters. As the saying goes, actions speak louder than words.
There are numerous forms on the internet of details you can create about your characters. But if you answer the main questions of what do they want, why, what’s stopping them and what they are and aren’t willing to do to get what they want, then you have a pretty good understanding of your character.