Ok, you want a free lesson. Because my knowledge is very valuable, I can't give away too much information for free, but I'll outline the basics of acting spastic.
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The main concept is bending. Your muscles will always try to bend. This means your hands are always near your shoulders, Your knees seem to like each other very
much and it's almost impossible to separate them.
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Next thing is random, fast and uncontrollable movements. Not all day long, but
just every now and then. This is what really freaks people out. Don't limit this
behavior to your limbs, but try to do it with your face as well.
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The third fundamental commandment is to use a lot of extra movements in addition
to the ones you 'planned'. To put it another way, add random noise proportional to
the intensity of the 'planned' movements. Never reach for something directly, always make some evading moves. Also keep in mind that working with your hands far away from your body is subject to more noise because then you violate the 'bend rule'.
That's about it for the fundamentals. It takes a lot of practice, so you better start quickly. Further info about the course is on the main page.