Reply to How much of DJing is preparation and how much of it is improvisation?

I've been trying to teach myself DJing for about 6 months and I believe I've gotten confused on something. Do DJs typically come up with ways to mix songs together totally on the spot, with no rehearsal at all? Or is it more of something that you sit with at home, trying out different ways to mix songs until you find one way that works and then doing that live? Basically, is DJing totally improvised or is it just running through mixes you practiced at home?