In fiction writing, characters that are interesting to readers are the ones who truly live and breathe on the page. Think about the last time you met someone face to face in real life. The first thing you took in about them was surely their physical appearance. Their build, their height, their eye color, or hairstyle. But even more than that, their physical essence. What did their appearance project about them as a person? What were their clothes like? A conservative suit and tie? Or a more care-free ripped jeans and a rock band tee-shirt? Or maybe it was a suit and tie as well as a visible rock band tattoo, making them a bit more mysterious and difficult to figure out with just one look?
Everyone we know, after all, is made up of many layers, many potential contradictions. Your characters should have those same layers and hints of contradiction. It goes well beyond describing what they look like, of course. Physical appearance is only the beginning, but the key here is that characters who are interesting to readers are the ones who make readers think, the ones who keep readers on their toes. Interesting characters keep us guessing about them from the moment we meet them on the page, and this is not just when it comes to villains. The best heroes are often the ones with the most layers and mystery. Are they genuine in their actions? Are they lying to us, the other characters in the story, or even to themselves? We will want to know.
When it comes to fiction writing, people will often recommend to "write what you know." Although this is helpful, it's often misunderstood. Your characters don't need to be you on the outside; meaning, if you work as a dentist, you shouldn't be cornering the market on characters who are also dentists. Writing what you know comes down to the essence of who you are inside. And, keep in mind, that who you are can change often.
Maybe there was a period in your life where you felt particularly confident in a certain situation. Conversely, maybe there was a period where you felt the complete opposite -- insecure and afraid. It's these feelings, and how you expressed them, both inwardly and outwardly, that you have to tap into, that you have to project onto your chartacters. Even if your characters' lives are far more dangerous and exciting than your own life will ever be, they still have feelings just as you do, just as we all do. That is what makes them interesting, because readers can see themselves in the characters, they can relate to them, even while the world and story circumstances might be wildly adventurous.
Readers want to go on that ride, they want to inhabit that space. That is what reading is, after all -- a chance to live, at least for a little while, in someone else's shoes. The more interesting, the more real, those characters are for us, the more willing we are to jump onboard for the ride.