The protagonist becomes a "ghost" in the room. Use this to reveal secrets about other characters that they only show when they think no one is looking.
If you're looking to create a story or scenario involving a time-stop train where the protagonist can freeze time and play pranks, here are some ideas and tips to enhance your narrative: The protagonist becomes a "ghost" in the room
Click. Not yet. Maybe later.
If you want to experience this fantasy through media, here are the best sources: here are the best sources: