Film stability and structure formation inside a liquid film containing colloidal particles are investigated by Monte Carlo (MC) numerical simulations and by analytical methods. The effective pair interaction between particles is calculated from the Ornstein-Zernike theory with Percus-Yevick closure. Consistent with the recent experimental observations and theoretical studies, these MC simulations reveal the phenomena of internal particle layering as well as inlayer structure formation. In particular, an ordered two-dimensional hexagonal structure is observed at a particle concentration of 37 vol% (instead of 43 vol% for the hard-sphere potential) when the effective pair interaction between particles is taken into account. Furthermore, the particles inside a layer "condense" due to the attractive depletion force which leads to the formation of voids. The formation of such void structures results in the formation of "dark spots" which have been observed in film thinning experiments. The calculated film structural disjoining pressure, created by the particles inside the film, is found to be in agreement with the experimentally measured force-distance curves using the surface force apparatus.