But the point is, why wouldn't the actor have known that Castle wasn't playing? I mean, if you think you're doing a stage-choreographed fistfight with someone and he socks you in the gut for real, that's gonna tell you it's not a performance to him, and you're going to have a very good incentive to try to bring a halt to the proceedings as quickly as possible. As soon as the guy realized that Castle wasn't stage-fighting, he should've called a time-out and dropped the act for his own protection.
I'm not saying the actor didn't realize it. I'm just positing that he could have been THAT good and believed he was not at all in any danger from an enlightened amateur such as Rick Castle, whose stage combat skills were rarely seen outside play-fencing with Alexis (or whatever he couldn't get away with while his mum was around teaching). Castle wasn't out to KILL the actor anyway, and the latter would be playing with that in mind. The fake guardian would have played it up as well as he could and then LET Castle "win", as per the script. I've actually done this sort of fake combat thing as part of a similar brand of treasure-hunting gig, though admittedly we fought with pool noodles.
Again, I know it's stretching it, but no more than some of the quirkier episodes of the show...