In this study, we attempt to clarify the difference between interdiffusion in thin-film and in bulk samples, and we have chosen the Ni-Si binary system for experimentation. A major difference is in the behavior of intermetallic compound formation; while the compounds form sequentially in thin films, they tend to appear simultaneously in bulk samples. The essence of sequential formation is the absence of the other stable phases during the growth of a specific one. We have used cross-sectional lattice imaging of the interface between a NiSi film and a Si to confirm the absence of NiSi2 at the interface. Other differences between the sequential growth of a compound in the thin-film Ni-Si samples and the simultaneous growth of several compounds in the bulk Ni-Si samples have been compared and discussed.