Liquid-polymer films sandwiched between two electrodes develop a surface instability caused by the electric field, giving rise to polymer structures that span the two plates. This study investigates the development of the resulting polymer morphologies as a function of time. The initial phase of the structure formation process is a sinusoidal surface undulation, irrespective of the sample parameters. The later stages of pattern formation depend on the relative amount of polymer in the capacitor gap (filling ratio). For high enough filling ratios, the final morphology of the pattern is determined by the partial coalescence of the initial pattern. The introduction of lateral-field heterogeneities influences the initial pattern formation, with columns nucleated at locations of highest electric field (isolated points or edges). The subsequently formed secondary columns have higher degree of lateral symmetry compared to the pattern formed in a homogeneous field. The nucleation of individual columns or plugs also dominates the pattern formation in the presence of an electrode consisting of an array of lines. The results of this study therefore allow us to draw the conclusion that the accurate replication of structured electrodes typically proceeds by the initial nucleation of individual columns, followed by a coalescence process that yields the polymer replica.