We develop a Green's function based formalism for calculating the tunnel conductance of a superconductor, where fluctuations can be incorporated through self energy effects. Since we are interested in describing non-equilibrium states with finite current flowing through the device, a non-equilibrium Schwinger-Keldysh field theory provides an appropriate framework. Our formalism does not depend on the specific functional form of the Green's functions, making it applicable to a wide range of fluctuating and strongly interacting SCs.