Polarising the L|L interface provides an in-built ability to control the kinetics of IET between oxidant and monomer redox couples in opposite phases and probe the mechanism in situ electrochemically. These two features elevate this work over studies that involve spontaneous interfacial reactions (polycondensation, polyaddition, self-assembly), that permit only rudimentary chemical control over reaction kinetics and a limited ability to study the synthetic mechanism in situ. Our work unravels the underlying thermodynamic limitations and mechanism of PEDOT thin film interfacial electrosynthesis that progresses along five distinct stages with time. This underlying mechanism will be broadly generic for all aqueous oxidant/organic monomer combinations that are thermodynamically compatible to facilitate IET within the Galvani polarisable potential window (where the IET kinetics are under direct external electrochemical control). Such an understanding will allow us to identify suitable oxidant/monomer combinations, opening the field of interfacial electrosynthesis to other technologically critical CPs beyond PEDOT, e.g., poly(3hexylthiophene-2,5-diyl), commonly known as P3HT. The dimensions and geometry of freestanding thin films possible by interfacial electrosynthesis are only limited by the interfacial area and shape defined by the electrochemical cell. Furthermore, we demonstrate that interfacial electrosynthesis is a local and mild process that requires minimal reagents (e.g., low oxidant concentrations) and is thus highly compatible with large scale thin film production. In this regard, future work will explore polarising the L|L interface chemically, by establishing a suitable distribution potential through ion partition between the two phases, to scale-up thin film production.