If the "magic" is going to occur on YOUR phone, you could use the app Tasker. It takes some practice getting the hang of using Tasker but there are resources online. I came up with a similar trick using this app. It is possible to run a task where you create what is called a "scene" which resembles your lock screen but it is really an image with fake buttons. It would be possible to set a follow up task where the phone "unlocks" itself after a certain amount of time after the spectator presses the fake buttons.