You were cast as an autistic teen in a play and you need to know their mannerisms or habits?

Short of attending a school for autistic children and observing first hand, check what others have done, such as Leo Dicaprio in "What's Eating Gilbert Grape", or the actor (I don't know his name) in the movie "Gigli", which is the only thing that could make me reccommend "Gigli" to anyone. This kid plays a very understated autistic boy, and gives the most impressive acting job in the movie.