  Black hole horizons interact with external fields when matter-energy falls
through them. Such non-stationary black hole horizons can be described using
viscous fluid equations. This work attempts to describe this process using
effective field theory methods. Such a description can provide important
insights beyond classical black hole physics. In this work, we construct a
low-energy effective field theory description for the horizon fluid of a
4-dimensional, asymptotically flat, Einstein black hole. The effective field
theory of the dynamical horizon has two ingredients: degrees of freedom
involved in the interaction with external fields and symmetry. The dual
requirements of incorporating near-horizon symmetries (${\cal S}1$
diffeomorphism) and possessing length scales due to external perturbations is
naturally satisfied if the theory on the non-stationary black hole horizon is a
deformed Conformal Field Theory (CFT). For the homogeneous external
perturbations, at the lowest order, this leads to $(2 + 1)-$dimensional massive
scalar field where the mass is related to the extent of the deformation of the
CFT. We determine the mass by obtaining the correlation function corresponding
to the effective field and relating it to the bulk viscosity of the horizon
fluid. Additionally, we show that the coefficient of bulk viscosity of the
horizon fluid determines the time required for black holes to scramble.
Furthermore, we argue that matter-field modes with energy less than $m_{\rm
eff}$ falling into the horizon thermalize more slowly. Finally, we construct a
microscopic toy model for the horizon fluid that reduces to the effective field
theory with a single scalar degree of freedom. We then discuss the usefulness
of the effective field model in understanding how information escapes from a
black hole at late times.
