study provides a unified analytical and numerical comparison between a classical clean stress-free thin-film interface and a strictly locally inextensible interface while retaining the same capillary.The resulting thickness evolution is coupled to a variable-coefficient elliptic equation for λ. Linearization about a uniform base film reduces the constrained mobility to one quarter of the clean-interface mobility; consequently, every nonzero constrained Fourier mode has one quarter of the corresponding clean-interface growth or decay rate, while the neutral condition, cutoff wavenumber, and most amplified wavenumber remain unchanged. Small-amplitude capillary levelling is retarded and the reported logarithmic slopes are consistent with the one-quarter linear-rate benchmark. In the molecular-force-driven calculation, the stated rupture threshold is crossed at 1.62 s for the clean surface and 6.48 s for the inextensible surface, giving a ratio of 4.00 for that specified setup. Gravity remains stabilizing in the supported horizontal geometry, but the constrained surface relaxes more slowly.