It is the discussant that stimulates civilized controversies, which are at the core of the conference.
One or more of the papers you have been asked to discuss might not be within your particular area of expertise, but do NOT start by stating this. Instead, act professionally and do your best to provide useful comments as best you can. Be clear, to the point, sharp but constructive, acknowledge significant contributions but focus on aspects where improvements can or must be made. Aim at providing that the author(s) with something useful to bring home. Enlighten, when possible, the audience with some new insight or reflection. Your comments should be given in judicious language. Disclose any financial or other interest you might have in the subject matter of the papers.  Let the Conference Organizers know immediately if you become suspicious of any possible fabrication of data, plagiarism or other kinds of scientific fraud relating to the papers you are asked to discuss.
In order that everyone has sufficient time to speak it is important that you exercise discipline, particularly time management. The following notes are intended to inform you of how each session will be organized. 
Please arrive at the appropriate room five minutes before the session is due to start. All rooms are equipped with black out facilities and a Power Point projector. Please note that if you have prepared some points on a PowerPoint slide you must arrive with your presentation on a USB memory device and load it yourself onto the machine provided in the room before the start of the session. It might even be a good idea to do so well in advance as not all versions of PowerPoint function equally well on all projectors. Solicit advice at the info desk or through a student assistant if experiencing trouble. 
Introduce yourself to the presenters, co-discussant and the chair. Give the chair your biographical details for use in introducing you. Ideally to assist the chair these should be in writing. One or two lines would suffice.
The chair will tell you at the beginning of the session how long your comments should last. This will inevitably vary between sessions depending on the number of papers to be presented. In a one and a half hour session with three papers you should aim to speak for no more than 12 minutes leaving time for your replies and for the floor.
We have asked session chairs to be very strict in terms of time management so that each discussant has an equal amount of time. 

By abiding to the simple rules your contribution will help support what we hope will be a productive, stimulating and enjoyable conference.