I cannot stress this enough: If you haven’t see The Last Jedi yet, for god’s sake DO NOT CLICK ON THIS VIDEO.
I’m not going to type out exactly what the spoiler is, but Boyega, having been through all the intricacies of Star Wars press tours once before, is suitably shocked.
Again, I won’t say what it is, but this is something that the movie has kept absolutely quiet (unless there’s some darkweb full of Last Jedi spoilers I know nothing about).