What do you mean by "substance"? Through all of my screeners and callbacks, I dont think I've discussed the "law" once.
Our experiences are different. I usually ask interviewees to tell me about an interesting legal issue they worked on over the summer. And I ask (reasonably easy) follow up questions. Someone who can clearly explain the legal issue they worked on and are reasonably thoughtful about their answers = impresses me on substance.
You can also impress on substance on all the other non-legal things we discuss. Most people have dumb canned answers to the normal questions -- why big law, etc -- and some really knock it out of the park. I don't knock someone down for having a bad answer but I do appreciate a very good answer.
How do we "wow" you on substance? Experience on resume? Intelligent and thoughtful answers? I would appreciate your honest feedback, as I'm not sure how to demonstrate intellectual firepower in a 30 minute law firm interview. I got into HLS and SLS, but my grades are likely bottom 15-20% of the class.
See my answer above. That said, and I say this to be helpful and not mean -- my firm doesn't hire below median, even at SLS or HLS -- so I don't know how helpful my answer is to you. My best advice is to get close to someone at a firm, ideally a partner, and get them to sponsor you.