My answer was this: I use a game I call, “What if?”. What if my main character, Skylar Robbins, explored a creepy old house and found it had a hidden floor? What would she find when she got there? What if she used an ultraviolet light in the attic and found a secret message written on the wall in invisible ink? What would it say? I like to put my hero in that type of situation and let my imagination go wild.
So my advice to aspiring writers is to interview your protagonist in your mind. Throw him or her into a sticky situation and ask how they would react. What if your main character discovered a tunnel hiding under some leaves in his backyard? Where would it lead? What if someone scary was hiding at the other end of the tunnel? How would he react? What if an unusual classmate begged your hero for a huge favor that was impossible to deny? How would she reply?
Put your characters into a challenging scenario and watch the scene unfold. The outcome may surprise you.