I think lying turns it from a prank and into a scam. A prank shouldn't require fraud to work.

It's like the show Punked, the best ones (thinking the kid from Malcolm in the Middle) relied entirely on the victim getting tunnel vision. If Franky had just been "yeah this is a prank" and it required someone to say "no, no its not" it kills the funny.

Remember those shows are edited, though.

Yes definitely, but it'd be hard to edit out a guy looking around for the cameras, and IIRC there was an episode of Punked where exactly that happened and they got called out, someone shouted "where the fuck is Ashton?" and ran around until they found a camera man.

That was the main reason they stopped. There was a hidden camera show in the UK (all I remember is a guy with a huge phone shouting in awkward public places like a library) that the same happened. Their third season was in random places in Europe because they kept getting called out.