Popping the question photos

A friend of mine asked me if I would take photos of him proposing to his girlfriend. I am a hobbiest so I have never done that before. I understand it will be different than an engagement shoot since she doesn't know about it. Has anyone done this before, what should I expect. Should I capture the moment and then afterwards get posed photos? Any help would be appreciated.

I did this for my neice. It was pretty fun. We went to downtown Chicago and we went to the park they had their first date at. Her sister and brother also went unknown to her. They held a huge sign off a bridge that said "will you marry me"? She was looking in the opposite direction, he got on one knee and I pointed at the sign and she turned around and saw it. It was magical. Afterwards I got a few shots of everyone together in the park. It was around Christmas time too.

Advice, talk about where he's going to do the proposal. The more room you have to get the shot the better. If you're cramped in a small room or restaurant you may not get a good shot.