But they're not entirely reactive without any planning. In fact, most of them focus on having a fixed set of requirements during any given phase/iteration.

The primary principle is to not start planning the *next* phase - or pieces you think you need - until you get to them.

I consider it similar to taking a road trip. If you're driving 250 miles to the beach, you probably don't plan every single turn, but you know the major routes, how often you need food/gas, and about how long it will take. As you drive, construction, traffic, nifty roadside things could distract you and affect your time/route and you may have to improvise, but you didn't start without a plan. And it's unlikely that you completely threw it out the window.