We present the infrared properties of -(BEDT-TTF)₂ Cu(NCS)₂ for E b and E c focusing on the evolution of charge conduction from high-T insulating to low-T metallic behaviour. Anomalous absorption manifested in a broad peak in the optical conductivity at high temperature is interpreted in terms of photon-assisted hopping of small polarons. A sharp Drude peak that develops at low temperature, together with large mid-infrared spectral weight in the conductivity, are attributed to coherent and incoherent bands of large polarons. The insulator-metal transition can be understood as a crossover from localized small-polaron to coherent large-polaron behaviour.