Before I really get into it, I want to begin with the main piece advice I would give to anyone, should they ever ask (hey, it could happen), who wants to improve his feedback (I also consider this to be the main piece of advice I would give a writer). See a movie outside your comfort zone at least once at week, or more. It doesn’t matter if it’s at the theater, on TV or through something like Netflix. But see what’s out there and get to understand that there are all sorts of movies that are sold, get made, and work.
But to begin, and this sounds kind of silly to say, but the main way to be constructive is to do the opposite of what people in my previous entry have done. It sounds simpler than it is.
First, don’t give feedback based on books or classes one has taken. Give the feedback based solely on your personal reaction to something. Some examples: don’t say the second act should start on pg. #, say that you were waiting too long for the other shoe to drop; don’t say there aren’t three acts here, say that there seemed to be some development of the story or character missing at certain points (and go into detail); don’t say there is no character arc, say that you got the feeling that the author was striving for a character arc, but that you were unclear what that was (if the author was striving for an arc; not all authors are).
Second, try and figure out what the author is trying to do and base your feedback on that, not on the way you would write it. If the author is trying to write a reactive character, give him feedback on how to help him achieve that. If the author is not trying to give his character an arc, then don’t try to find one for the character. If you don’t know what the author is trying to do, there is nothing wrong with saying that (I have said just that in the past). For example, it’s ridiculous to say that the central characters to Leaving Las Vegas or Richard III aren’t sympathetic heroes when that’s not what the author is going for. You may also have to ask, though this isn’t always an option. Loglines can be very useful here.
Third, if you are uncomfortable with certain types of screenplays and certain subject matter, then you probably shouldn’t be doing coverage or giving feedback on it (you certainly shouldn’t be doing coverage or feedback for a living).
Fourth, don’t worry about whether a screenplay has a big audience or whether there is a production company out there for it, unless the author has made it clear that he is trying to write a screenplay for a particular audience.
Finally, do not be rude. There are some people out there who confuse rudeness with frank criticism. They say the most awful things to people about a screenplay without any thought of how the writer might feel. You know these people. The dead giveaway is that when you suggest maybe they are being too rude or negative, their defense is that they’re only being honest and that if the person can’t take the heat, he should get out of the kitchen. It’s somewhat sad and pathetic on their part; for these people, you just need to avoid them.