How to be a discussant
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.