Using forward recoil spectrometry and atomic force microscopy, the phase evolution of a critical blend thin film of deuterated poly(methyl methacrylate) (dPMMA) and poly(styrene-ran-acrylonitrile) (SAN) is found to develop by three distinct stages. During the early stage, dPMMA-rich wetting layers rapidly grow at the air/polymer and polymer/substrate interfaces. A hydrodynamic flow mechanism is proposed based on the scaling of the layer thickness with time, t(-1), and the direct observation of an interconnected, bicontinuous morphology across the depletion zone. The lateral wave number of this morphology grows rapidly as t(-1) but slows down to t(-1/3) when the phase size approaches the film thickness. During the intermediate stage, the wetting layer thins and, concurrently, dPMMA-rich domains spanning the SAN-rich middle grow as t(-0.41) in good agreement with an interfacially driven growth model. During the late stage, these capillary fluctuations eventually cause spontaneous rupturing of the middle layer resulting in an interconnected 2D network which eventually coarsens into isolated SAN-rich droplets encapsulated by a thick dPMMA-rich wetting layer. The surface roughness increases rapidly initially, reaches a constant value, and then increases at a much faster rate than that predicted by a trilayer model based on capillary fluctuations.