How to prepare as reader

Read the paper!

Keep a list of questions that occurred to you while reading. Attempt to answer the questions yourself (by thinking, by keeping to read, by looking for answers in other material / online), and write them down. Others might have had the same questions.

Your job as discussant is to help the presenter bring out the central ideas of the paper – by asking questions that give an opportunity to explain something, or by providing additional information or understanding of the material.

Your job is not to just sit there and expect knowledge to be transferred to your brain.