Quantum effects of fields on curved spacetimes may be studied in the laboratory thanks to quantum fluids. Here we use a polariton fluid to study the Hawking effect, the correlated emission from the quantum vacuum at the acoustic horizon. We show how out-of-equilibrium physics affects the dispersion relation, and hence the emission and propagation of correlated waves: the fluid properties on either side of the horizon are critical to observing the Hawking effect. We find that emission may be optimised by supporting the phase and density of the fluid upstream of the horizon in a regime of optical bistability. This opens new avenues for the observation of the Hawking effect in out-of-equilibrium systems as well as for the study of new phenomenology of fields on curved spacetimes.