Copyright question about displaying short excerpts from a documentary online?

Guess this is as much a moral as a legal question.

So, say I have a documentary bought on iTunes, and I discuss something I picked up in it here on Fluther.

Now in the course of the ensuing discussion I’d find it helpful if others could see a certain part of less than 2 or 3 minutes.

Would it be okay to capture this very short part and put it on YouTube just for illustration’s sake?