If in the process of writing your very first sentence of your novel; it is important to make your main character believable. Not only them, but the protagonist and supporting cast.
The advice I was given when I first started writing was to ask your main character questions. What type of food do they like, where they went to school, are they married, single or any question you would like to ask them to bring them to life in your mind.
I opened a separate word document filled with questions and answers. It was easy to understand how they would react in certain situations because I had already asked that question.
Names are also important.
Research to find out the popular names of the year they were born. Think about where they were born, and what their background is. Yes, your characters are fictional, but they need to be believable too.
If your characters feel real to you, then they will feel real to the reader. They will empathise with their journey and cheer them on from the side lines. Cry with them when life knocks them back. Want to turn the page to find out if they succeed or fail.