High-aspect-ratio (HAR) pillar arrays offer large surface area, well-defined surface topography, and large mechanical compliance. In this Prospective, we showcase micro- and nanopillar array systems that exploit the responsiveness and/or harness the mechanical instabilities for myriad surface-mediated applications, including tunable wetting, adhesion, optical properties, and actuation. In each application, we start with biological examples with HAR pillar structures, and discuss strategies to fabricate responsive HAR polymer pillar structures. We then discuss approaches to tune the surface topography, such as via bending, tilting, expansion or contraction, and collapsing of the pillars, and the resulting change of surface and optical properties, and their dynamic actuation. In each system, we discuss the controllability and recoverability of pillar deformation.