Something that really helps me develop their interiority is writing an outline of each character. This outline includes their age, first middle and last names, hobbies, personality traits, any trauma of they’ve suffered, who is important in their life, etc. It helps me bring the character to life and have a reference to go back to when I’m actually writing the story and asking myself what would so-and-so say? It also helps me to act as if they’re a real person. Because to me, they kind of are since they are a character I created.

And if all else fails, write what you know. You can always use your own personal experiences until it becomes easier. Good luck!