Their brain forces them into a quick-quick after two or three slows. Count S and QQ randomly to real foxtrot music so they can hear it in the music. All possible combinations of directions would be included. Then I stick to the school figure. In this case, for practice, I would talk and agree on the plan.
Spontaneity can come later.
It's easier to practice things in isolation. When practicing a series of slow walks, we both can concentrate on the walking action, lead and follow, rise and fall. When practicing a series of side rocks we can concentrate on that feeling also. When we practice changing direction from forward to side we can concentrate on the gathering, momentum change or whatever. Each step encompasses many techniques, movements and positions.