Taken from superherohype.com:
Speaking with EW about the crossover, executive producer Andrew Kreisberg said they didn’t want to wait too long to see both heroes (and their helpers) team up.
“It’s really going to be an adventure with the Arrow and Flash on both episodes. Watching the two teams come together and fight alongside each other, it’s one of the most fun parts. We just don’t believe in waiting. We really believe in accelerated storytelling and especially for those first nine episodes of the season—for both shows—hopefully we’ve designed it so that none of these [make you say], ‘Well, I missed that one, it’s fine.’”
Kreisberg also teased the big bad for the upcoming third season of “Arrow,” saying:
“Slade had a very specific agenda—he was out for revenge and had set up this elaborate five-year plot. What’s interesting about the villain in season three is that he doesn’t necessarily disagree with [Oliver]. He doesn’t have any personal animus towards the Arrow, and he actually in some ways has a very similar worldview. [But] the Arrow is thinking too small.”