It is considering that SJ had his " gut-feeling " pretty well straightaway after JB had been questioned and the relatives " knew " days after.
I guess, if you've ever 'dressed' a stage, and/or those on it, you'd understand that it's all about illusion. JUST enough is done to make the audience believe that what they're seeing is real, after which, if the work requires a narrator, that person will tell the audience what they're about to see played out. I think Jeremy managed it all remarkably well.