How do you get to know your characters well enough to feel confident writing what they do and say?
The great director Federico Fellini had a particular method. He said, "Some people get a quick handle on things by means of words, by sensations. I design. I sketch the traits of the face, the details of the costume, a person's attitudes, anatomy. That is my way of getting close to a film I'm making, of understanding what it is..."
You don't have to be an artist to use this method. Nobody else needs to see your crude sketches, the important thing is that they help you understand the character.
If you don't draw at all, you can look for images online that inspire you.
For instance, let's say your protagonist is a young man.
I searched for: Google images young man. What came up at the top of the results page was '1000+ amazing young man photos' (these are free stock images). If you scrolled through these images I'm sure there'd be one that clicked, that made you think, "That's my protagonist!"
You can do the same with clothing, settings, and the other elements of your story.
Give it a try and let me know how it works for you, either in the comments or via email at [email protected].
Good luck!