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It could be either. I've dated some women that I found appealing from the first time I met them. I've also dated some that I was friends with for a while before the romantic interest developed.
In the long run, I like familiarity. I like to know my partner very well, but that has to start somewhere and sometimes it starts with meeting someone new that seems interesting as a potential dating partner from the beginning. Other times some of the familiarity develops before the interest in her as a dating partner does.