A story is usually a vehicle for characters to move from a starting point to a life changing end point. The level of detail you bring to the characters, has a major impact on how readers connect to them. Traveling through the most awe inspiring world building without connecting to the characters is no more than a fancy travelogue.
I like to start with the point of view (POV) character. Who will readers experience the world and the story through? This main character is usually my protagonist, so I want the audience to be sympathetic to this character. In Fantastic America, there are three POV characters. Ashley Monahan, the protagonist, Daniel Forrester, the antagonist, and Jerry Farmer, the villain. Each serves a different function, but needs an intimate level of detail.
The process I go through for each of them is the same. I have a rather long form with physical details (height/hair/body type), social details, connections to other characters, likes/dislikes, and room for a biography. I can add to this over time, but it gives me a good general start. A big part of a novel is their personal journey, so some of the details about a character should change over the course of the story.
For main characters especially, I like to find a picture of someone who fits my idea of their look. If I’ve used a real person as inspiration for them, I’ll use a picture of them. When I’ve invented the character from scratch, I use AI face generators that no one else will ever see.
The final layer I add before writing this character into motion is to give them quirks or idiosyncrasies that set them apart. This can be a simple as their body language or as profound as a health issue. Pretty is rarely interesting. So even my characters who are physically attractive have some inherent flaw or problem readers experience from the inside. Some mental falsehood they believe, emotional trauma they relive, or a loss that isn’t visible at all.
Less detail is needed for non-POV characters, but I never write them off entirely. Early in my journey to write the magic unleashed series, I started writing short stories in the same world. I found a need for secondary or supporting characters to go off on adventures without those main characters.
They became main characters of their short stories. I had their bones already, adding flesh using this method was painless, and let me get to the writing that much faster. So for me, any named character gets at least a one line entry in my world building character list.