How to be a discussant There is another research group at another university in my town. They asked me to join one of their monthly brown bag meetings and to serve as a discussant. They have invited an external speaker and he will present something that is related to my own research. 

I received the presentation slides in advance and I have read two related papers of the speaker to prepare. Although I do have some questions and points to discuss, I am not exactly sure how to go about this. I have never done this before and I have only once witnessed such a thing at a conference but thought the discussant there wasn't a good example.
So I was hoping for some general advice and experiences. Some questions are:


Should I prepare some slides myself?  
How long is this kind of discussion usually?  
Should I try to engage the audience or is it more of a one-on-one between me and the presenter?
Since we don't know your field, I'll add generally-speaking for my field, economics:


Slides are common for discussants at large, formal conferences, but never for more informal brown bags
Again, at conferences discussants tend to go longer, 10-15 minutes.  At a brown bag, five minutes or less seems about right
You should engage the audience in the form of opening up a thoughtful insight or question on the paper, one that the audience can then pursue in Q&amp;A.  Usually it's presented in the form of you talking to the presenter though.


But those are just broad strokes, the key here is I think you should just ask.  You don't have to mention you've never done this before; just ask how long they want you to talk for and whether you should prepare anything.