Polymers have been used to stabilize interfaces or to tune the mechanical properties' of interfaces in various contexts, such oil emulsions or biological membranes. Although the structural properties of these systems are relatively well studied, instrumental limitations continue to make it difficult to understand how the addition of polymer affects the dynamic mechanical properties of thin and soft films. We have solved this challenge by developing a new instrument, an optical-tweezer-based interface shear micro-rheometer (ISMR). With this technique, we observed that the interface shear Modulus, G*, of a dioctadecyldimethyl-ammonium chloride-(DODAC) monolayer at the air-water interface significantly increased with adsorption of polystyrenesulfonate (PSS). In addition, the viscous film (DODAC monolayer) became a viscoelastic film with PSS adsorption. At a low salt concentration, 10 mM of NaCl in the subphase, the viscoelasticity of the DODAC/PSS composite was predominantly determined by a particular property of PSS, that is, it behaves as a Gaussian chain in a theta-solvent. At a high salt concentration, 316 mM of NaCl, the thin film behaved as a polymer melt excluding water molecules.