A good thriller begins with a death, right? So who do you kill off and how do you make your dead character relevant to the story? One way is through short flashbacks.
For example, a child sees something that reminds her of her dead or missing parent. The emotions she experiences are important parts of the story.
Perhaps a colleague becomes upset when a new hire is assigned the missing person’s desk. In the process of settling in, anything left behind is packed up and put away.
Your task is to write a story in which a character is either dead or missing. Your protagonist experiences flashbacks of times shared, places gone, things purchased that remind her of her parent/child/coworker.
Don’t make the flashbacks too long for fear of pulling readers out of the story. Short and quick is better.
Use a combination of narrative and dialogue. Begin with a strong setting that puts the protagonist in a situation that is poignant.
Have fun with this one.