There's a claim repeated in many places online that his IQ was 86 but I don't know the source of this. I fully understand the limits of IQ testing as a measure of intelligence, but when I watch interviews like this I notice he does seem incredibly inarticulate and... well, stupid. The interviewer is forced to feed him every answer.

Is that how he was like with other people who knew him? Was he capable of having a normal intelligent conversation with anyone? Was he able to elaborate on all of the ideas behind his art?