D. Campbell (home page)
Discussant for a single speaker (e.g., you are the discussant for a visiting speaker to the college).
The discussant can play different roles. here are a few models (some better than others):
the bridge/transition from the lecture to the Q&A: your task is mainly to shift the tone and focus from the lecturer to a discussion.
the translator/clarifier: you help the audience see the main points, the threads of the argument, clarify some opaque language (useful for an arcane, challenging lecture)
the provocateur: you challenge the lecturer, question the argument or evidence or methodology, play devil’s advocate, etc.
the complementer: you sing the praises of the speaker
the moderator: you are simply there to say a few kind words but then shift to facilitating the Q&A part of the evening.
The first questioner: you frame your comments around posing a few questions -- in a way, to get the ball rolling on the Q&A session.
The alternative speaker: you take the opportunity (usually inappropriately) to give your own answer to the question, to sing about your own research and ideas.
In general, I would recommend doing a combination of a few of these: e.g., 1, 4, 5, 6, and a bit of 2 if needed.
Discussant on a conference session (e.g., 3-5 papers).
This is somewhat different in format than being a discussant for a single speaker lecture, though the tone/approach may be similar:
Be a good timekeeper (a tight ship keeps everyone happy). [sometimes there is a separate moderator who takes care of timekeeping, which allows you to wholly focus on the presentations.]
Generally start with some general comments about common themes across the papers. (This is especially welcome if the session contains a seemingly disparate set of topics and the audience is a bit uneasy about the apparent lack of a central theme.) [e.g., “Though these four papers seem to address very different aspects of planning in very different case studies, they all have at their core a concern about governance and the limitations of the local state to ….”]
Then offer a few sentences on each paper (e.g., 1-2 minutes each). Aim to say at least one positive thing about each paper, then one gentle suggestion (often areas to explore further, more in depth, etc.).
End with a few general comments/questions about the papers as a group, opening the discussion up to audience, welcoming them to ask questions.
Most people should send you their paper in advance, but some don't (and you just base your comments on what they say at the session.)
How long? I try to keep to 5-7 minutes, but sometimes run longer. (You don't want to take all the remaining time in the session, since you want to leave time for Q&A).
See also this humorous view of conference culture politics.