Control over the physical properties of nanoparticle assemblies at a liquid–liquid interface is a key technological advancement to realize the dream of smart electrovariable nanosystems. Electrified interfaces, such as the interface between two immiscible electrolytes solutions (ITIES), are almost an ideal platform for realizing this dream. Here, we show that the Galvani potential difference across soft interfaces can be effectively used to manipulate: (i) the reactivity of gold nanoparticle assemblies through varying the Fermi level (both chemically and electrochemically); (ii) the location distribution of the nanoparticles at the liquid–liquid interface. In the first case, in addition to our previous studies on electron transfer reactions (ET) across the ITIES, we used intensity modulated photocurrent spectroscopy (IMPS) to study the kinetics of photo-induced electrochemical reactions at the ITIES. As expected, the direct adsorption of gold nanoparticles at the interface modifies the kinetics of the ET reaction (so-called, interfacial redox electrocatalysis), however it did not lead to an increased photocurrent by “plasmonic enhancement”. Rather, we found that the product separation depends on double layer effects while the product recombination is controlled by the Galvani potential difference between the two phases. In the second case, we demonstrated that polarizing the ITIES caused migration of gold nanoparticles from the middle region of the cell to its periphery. We called such systems “Marangoni-type shutters”. This type of electrovariable plasmonic system did not experience diffusion limitation in terms of the adsorption/desorption of nanoparticles and the entire movement of nanoparticle assemblies happened almost instantly (within a second). It opens a fresh view on electrovariable plasmonics and presents new opportunities to create smart nanosystems at the ITIES driven with an electric field.