A great way to build realistic character development is to interview your characters. There are many different ways to go about interviewing a character. You could do a very basic overview (from the outside, looking in) and answer basic questions in a simple character template format.
It’s like you’re filling out the census for them, except you don’t have to worry about bubbling your answers in. . But, let’s say your character goes beyond that straightforward template, especially if it’s one of your main stars. What would you ask yourself about your character?
Do they have a goal?
What’s their biggest flaw?
Do they have a religion?…etc.
You get the idea. We’re starting to dive a bit deeper here. Now you’re starting to take a step back and are starting to put yourself in their shoes to better understand their fictional life. They’re starting to come to life…THEY’RE ALIIIIIIVE!
Interview your character like I would interview Chris Hemsworth (as Thor) about Asgard or Gal Gadot (dressed as Wonder Woman) about forming the Justice League. Sorry (not sorry, it’s very relevant because of Infinity Wars) about the comic book/film references… moving right along!
Envision your character sitting across from you. Catch a glimpse of their mannerisms and body language in reaction to the personal questions you may ask.
Are you lying to yourself about something?
How did you meet your best friend?
What do you want your tombstone to say if you don’t survive this mission?…etc.
The more realistic the character becomes, the more you’ll question whether they exist or not. Make your reader believe!