The interfacial region between two bodies of turbulent fluid was investigated through simultaneous particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) experiments in the far wake of a circular cylinder. Interface conditioned plots of enstrophy revealed the existence of a turbulent/turbulent interface (TTI) where the enstrophy adjusts itself between the two regions. An enstrophy jump was present even in the most extreme cases of subjected free-stream turbulence. Further analysis of the TTI through the lens of the enstrophy budget equation highlighted the altered roles of inertia and viscosity in the vicinity of the TTI. Unlike the turbulent/non-turbulent interface (TNTI), the inertial term is largely responsible for enstrophy production in the outer regions of the interface, whilst viscosity plays a much more subdued role. The global effects of free-stream turbulence on entrainment behaviour was investigated through the measurement of the mean entrainment flux. It was shown that an increase in intensity of the free-stream turbulence acted to reduce the mean entrainment flux into the wake. Length scale of the background turbulence on the other hand did not greatly influence entrainment behaviour in the far wake of a circular cylinder.