Hi, this is Jessie McEntee. I've worked at WWW as an instructor for almost eight years. I'm sometimes asked for tips on giving great feedback during a workshop. I'd say the following: Learning to give great feedback is an art, and you'll develop a style of your own as you take more and more workshops. Here's a quick list of pointers.
1) In the beginning, perhaps concentrate on your emotional reaction. Keep your feedback positive and focused on what's working. Your response might sound like, "I laughed at the section when...." (Assuming you feel confident the author intended this section to be funny!) You could say something like, "I found it really touching when...."
2) You might also think about what you can infer from the characters' actions and words. This relates to one of our favorite sayings as writing instructors: "Show don't tell." Perhaps you can tell that a character is an optimist based on her plucky attitude towards a setback or the way she speaks. This is absolutely worth noting, which brings me to another point. Like great writing, great feedback is specific and concrete.
3) Over time as you become more comfortable, think about issues of craft and how they're working in the pieces you're hearing. Again, try to be specific and concrete. Perhaps this sounds like, "Your setting is working well because..." or "I can differentiate who's speaking well within your dialogue because...."
What's NOT helpful in terms of feedback? By and large, we discourage feedback based on the content. A comment like, "That could never happen" isn't the most useful one. Someone's going to write whatever inspires them. I'd say the one exception is for something like a medical condition or a description of an exotic place, say, because the writer likely does want to make this plausible even in a fictional piece or a novel (unless they're writing fantasy or sci fi). You could always say something like, "I have personal experience with that condition or locale, and I'm happy to discuss what I know offline if that's of interest for your research." It's not your job as a listener to fact check everything you're hearing! Also, we as authors feel vulnerable after sharing our work. Try your best to be kind and encouraging.
In general, keep workshop time focused on lessons that benefit everyone. Direct your attention to character, setting, dialogue, tension, pacing. Does the scene appear to reach a climax or a tipping point? Does the author reveal something new that you didn't know before? Can you tell that a protagonist is evolving over time? Can you discern what motivates the protagonist deep down? Do you connect with one or more characters? Why?
Finally, there's no shame in saying something like, "I agree with everyone else's feedback, so I don't want to take up time repeating what's already been voiced."