The behavior of a binary polymer mixture was simulated on a cubic lattice over both the miscibility and immiscibility regions. The number and distribution of interactions in the mixture were found to be different from the mean-field picture; however, the observed phase behavior agrees with that predicted by the mean-field theory and is not affected by the observed concentration fluctuations. The relationship between the phenomenological x parameter and the heterosegmental interaction energy was investigated. Polymer chains show nearly ideal behavior, even for strongly interacting mixtures; this simplifies the theoretical treatment of polymer mixtures analogous to homopolymer melts.