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I was made an offer today on the spot after call back. It wasn't my top choice, so I didn't want to accept it immediately. I don't think I handled it well. How would you say you will respond to such an offer other than expressing gratitude and excitement?

If it's a NALP firm they are required to keep the offer open. Even if it's not a NALP firm just say you very much appreciate the offer and ask if you can think about it.