GRBs (Gamma-Ray Bursts) are one of the most powerful and farthest explosions in the universe. One of the crucial questions plaguing the study of GRBs is the mysterious mechanism responsible for the prompt gamma-ray emission. In particular, the non-thermal nature and the softness of the observed spectrum are unresolved issues. To tackle these we perform Monte Carlo simulations of radiation- matter interactions in a scattering dominated photon-lepton plasma. In our model, the plasma (initially in equilibrium) is driven out of equilibrium by energy injection mimicking the effect of energy dissipation via shocks or magnetic reconnection. We show how equilibrium restoration leads to transient yet observable non-thermal features such as power-law tails, high energy bumps etc. On comparing our synthetic spectra with observations (Band Spectrum) we predict spectral correlations and conclude why photon-rich plasmas are a better GRB plasma candidate than pair-enriched plasmas.