Sam playing bad cop
You know how Sam was so horribly awkward last week when playing bad cop with the restaurant employees?
You notice how scary and effective he was playing bad cop with the Wiccan woman last night?
The difference?
Dean's life was at stake.
Sam can't play bad cop effectively unless Dean's life is on the line. In which case, it's not playing.
This just reiterates something I mentioned in my "Plucky Pennywhistle" commentary.