  The time-dependent exchange-correlation potential has an unusual task in
directing fictitious non-interacting electrons to move with exactly the same
probability density as true interacting electrons. This has intriguing
implications for its structure, especially in the non-perturbative regime,
leading to step and peak features that cannot be captured by bootstrapping any
ground-state functional approximations. We review what has been learned about
these features in the exact exchange-correlation potential in time-dependent
density functional theory in the past decade or so, and implications for the
performance of simulations when electrons are driven far from any ground-state.
