In this talk, I will present a novel geometric method, inspired by similar computations in AdS-CFT, to obtain the open system dynamics of a localised observer in de Sitter spacetime. This dynamics is encoded in a Schwinger-Keldysh effective action that describes the observer's radiation reaction as well as fluctuations associated with de Sitter Hawking radiation. I will discuss the computation of such an action for observers coupled to scalar and electromagnetic fields. We will find that the action provides a Langevin description of the observer at cosmologically long time-scales. In contrast, the near-flat space expansion reproduces the Abraham-Lorentz-Dirac force with cosmological corrections.