This present work reports results of an experimental campaign in a 20 mm internal diameter tube to comprehend the interfacial evolution of a fully developed Taylor bubble to an annular bubble at the inception of an annulus. The phenomenon has been observed by using a high speed photography camera and has been further analyzed by using image processing tools. Interfacial evolution is found to be a complex phenomenon with various physics rich processes occurring simultaneously in six stages, namely, retardation of fully developed Taylor bubble, plateau formation, doughnut shape formation and nucleation of lobes, preferential rise of leading lobe and retraction of lagging lobe, thread formation of lagging lobe, and finally, manifestation of an annular bubble. Effects of fluid viscosity and eccentricity on hydrodynamics features, such as annular bubble rise velocity, film thickness, nose shape, and reacceleration of the annular bubble are investigated in detail.