Fair Witness-originating from the book 'Stranger in a Strange Land' by Heinlein, Fair Witness was a made up profession, and their job was to record everything they heard/saw. What it means play/be fair witness is to, well take questions people ask you somewhat literally, as seen in the example. Yes, you can do something, but that doesn't mean you will-and that can lead into you will/would do something but not right now because no time was specified. This pretty much means messing with peoples' words/questions until they ask you in a very specific way.
James: Okay, can you pick up all of your crap?
Michael: Oh, I'm sorry-did you want something?
James: Stop playing fair witness a pick this stuff up!