@welcome2atlantis It's all about how natural one acts. Just like veteran cops will immediately notice when an ordinary joe is trying to hide something suspicious. Su Xiao is an idiot, so he can do only a few things, but those things he does fluently precisely because inelligence would only hamper performing them. Ming Feizhen's problem was that he tried to copy an idiot's act, yet he's not an idiot, so it turned out to be difficult. You often see this plot element in stories (and also in real life) when a naive, innocent person is sent to do something without realising there could be any complications. Their act will be perfectly natural, as long as it's something they can do. Of course pro criminals can also pull it off due to their experience. In the simplest example case a person might be sent to deliver a false message. If they don't know it's a lie and suspect absolutely nothing, there will be nothing amiss in their behavior.