Thanks! I doubt it was fully mapped out -- though Joss must have known that Darla was the ex and Buffy was the love interest since I figure season one pretty much got worked out as a whole. (Indeed, they reshot scenes from episode one towards the end of the season's shoot, didn't they?). I think the real knack was to take what had been laid down and play off of it -- see, e.g. the balcony scene echo in season 6.

The movie is coming to Netflix streaming soon, so I will finally watch it!