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An environmentally friendly and commercially available polymer, ethylcellulose (EC), was found to be effective for water removal from water-in-diluted-bitumen emulsions. In a previous study, atomic force microscopy images of bitumen films transferred from the water–toluene interface onto a silicon wafer revealed progressive disruption of bitumen films by increasing the addition of EC. The in situ micropipet experiments, on the other hand, demonstrated the flocculation and enhanced coalescence of two approaching water droplets at low and high EC concentrations, respectively. In this study, we investigated the effect of EC addition on compressibility, composition, thickness, and surface properties of the interfacial films formed by surface-active components of bitumen, aiming to understand the interfacial behavior of EC at the water–diluted bitumen interface. The pressure–area isotherms of Langmuir interfacial films indicated a transformation of a rigid interfacial film formed by surface-active components of bitumen to become much more compressible in the presence of EC. The polarized infrared spectroscopy analysis of the Langmuir–Blodgett (LB) interfacial films proved the adsorption of EC at the water–oil interface, while the thickness measurement of the film indicated the displacement of interfacially active materials by EC. The thickness and contact angle measurements of LB interfacial films revealed an irreversible nature of EC adsorption at the interface. In conclusion, EC is able to irreversibly displace/disrupt the interfacial film formed by surface-active components of bitumen at the water–oil interface and increase the compressibility of the interfacial film, promoting the coalescence of water droplets.