How did you develop a friendship with someone of the opposite sex?

The question of whether men and women can be friends shows up regularly, but I’d like to take a different angle. I’m wondering how it happens, and even more importantly, what people are thinking while it happens.

I’m wondering if there’s always a stage where the question of taking it into a different realm (love and/or sex), and if so, how do the friends navigate thought that? Or does the friendship develop the same way as friendships with folks of the same sex, and love and sex are never an issue?

In either case, how does it happen? If you have experience with this, please tell us how you got into it, and what were the problems you faced and how did you overcome them?