Mental disorder

In class we have to have a partner act out any disorder we like and we do an 11 page intake interview with them. My partner happens to be the guy who in my skills class would only say "no" to each question I asked. Such as
me: "What parts of school DO you like?"
him: "No!"
me: "How do you feel about your teachers?"
him: "No!"
I guess that makes it easy, but it's frustrating. So he's my partner again, and he promised to not do that, but I have a sense of dread because of the mental illness I gave him - dementia. I think dementia would be fun to act out, but I'm a little afraid of what he'll come at me with. What's worse is that when I asked him what he wants me to be he just smiled and said "I'm still deciding." Hopefully he isn't revengful!