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So, here's the specific situation...
Met a girl last week at an acting class. Said hi just before the class but not a lot of time to chat during, and she needed to leave right afterwarss. Saw her again this week and there was a bit of time before the class started. Couldn't work out how to start conversation.
I think I need a re-opener or something.
Don't overcomplicate things... Saying something is always better than saying nothing. (HI, Hello, How are you doing?) There is nothing wrong with using these to start a conversation. When in doubt using something like,
"Hi, You know I see you in class and I'm curious as to what you're all about. I'm (name)." 
CHAT