As a discussant, should I discuss my points with the presenter beforehand? There is a good question about how to be a discussant here, but it does not cover my question. 

I was chosen to be a discussant for an upcoming seminar (actually, it was proposed to me by the same author I am discussing). I have several comments to make about the work, which I will put on slides. Should I talk the author through them before my presentation, or perhaps send him/her my slides beforehand? Or is it well understood that discussions are "surprises" and presenters should be able to address the discussion "on the spot"?

I have no intention to put the presenter into trouble, but perhaps, for the sake of the audience, it is better if I send the slides/discussion beforehand, so the quality of the answers might be higher. But I am not sure this is standard. Any ideas?
I would go with the second option; send the slides to the author beforehand. This will help the following:


The author would be prepared in terms of the questions and topics he is going to get the questions from. He would think on a lower level and add more clarity to his answers.
Give the author a higher confidence to defend, if there seems to be any surprise.
If he feels, he can ask you for additional clarification beforehand.