Hmm, it seems like the mentality towards the way we would view these types of people should change. As the performer, we have to realize that people asking the above questions probably don't know how the tricks are done, so those questions are normal to them. If you do a false cut/shuffle and ask "Was that fair?" (or whatever line you use), that opens you up for a spectator to comment "Can you shuffle again?". When you fool someone (and that "is" what magic is doing, just in an entertaining manner) they are going to want to figure out how you did it. If they didn't, their reactions would be fairly lame, however some people try to solve the puzzle in varying degrees of directness. Imagine a spectator you just performed one of your personal best tricks for, and at the end they just say, "Ok, cool" with little amazement or wonder of how you did it. The question of "How did you do that?" is one that we should want to hear, but often naturally comes with follow-up questions by the spectators to try and answer their own questions.
Now there are some, if not many instances where a spectator is obviously heckling, like the "Oh I forgot my card" guy with a grin on his face (some people actually do forget their card, if you happen to get someone like me who has a hard time focusing on one thing at a time, lol), or the guy in the background yelling trade secrets during the performance. We as performers have to ensure we are aware of the difference between the legitimately curious spectator, and the heckler, because it's easy to slip into a mentality of disdain towards all spectators.
As others have mentioned above, a humorous response to their question is best, one that will make the other spectators laugh, and psychologically disregard that persons future questions. In the case, a heckler knows when he/she is heckling and could either become more belligerent or just be quiet. An honestly curious spectator will laugh along and continue with the viewing, with little to no feeling of unfairness towards them. Another thing when it comes to handling of these situations, we have to understand that it is not necessarily about WHAT you say, but about HOW you say it. Tone and inflection are important. Think about times in your past when someone used the words "Please" or "Thank You" in a comment or question towards you, and afterward you didn't feel very pleased or appreciated, but more so pissed off. For examples sake, take these (pardon my acronyms) "Would you PLEASE STFU?" or "THANK YOU for killing me with you" (says a passerby as you smoke in a public area). Those are just quick examples of my meaning.
You could start of your performance by stating "I am going to perform some (substitute your word here) for you right now, please hold your questions until the end, but feel free to be amazed. Acceptable facial expression are (exhibit a few expressions of surprise, show and amazement), if they laugh, great, if they don't, throw in a "just kidding about the expressions, it's your choice ;)" (Look at reaction videos to magic, and pick some). This is attempting to establish your directives to the audience, but adds a comical twist onto it so they don't feel like you are disrespecting them. If you can get them to verbally or non-verbally commit to holding their questions till the end, it gives you room to stuff any heckling questions during the performance without seeming rude, plus they will be conscious of not asking even smaller questions like "Can I inspect that card?". From my personal experience with human interaction (hehe), I've noticed that if you can make them laugh with you, they are less likely to try and bring you down or embarrass you.
A caveat to the above, I notice people making similar mistakes in communication with others in regular conversation. So for a performer to make the same mistakes or miscalculations of the audience, can be detrimental because it is on you to win them over, not other way around. These are also just my opinions and experiences, and in no way am I trying to portray myself as an expert on the topic. (See, that last sentence is meant for those who view themselves as experts and in disagreement with me, to be less critical towards my point of view, because now they know I am not challenging their expertise). It's all about winning peoples minds over with the way you communicate with them.
I think I have typed enough, but didn't really scratch the surface on the inner workings of the topic, because it goes way to deep to discuss forums, lol...TOODLES!!!
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