We explore here the decisive role of film thickness on Casimir and electrostatic forces from topological insulating Bi2Se3 films. The forces were measured for the Au-Bi2Se3 system under ambient conditions to simulate realistic applications. It is shown that the electrostatic forces associated with a nonzero contact potential of Vo∼200mV played a dominant role, and could not be compensated for thinner films (≤20nm thick). However, for thicker films the measured force, after voltage compensation, started to approach the genuine Casimir force in very good agreement with Lifshitz theory. Our results point out that any gradual deviation from theory for thinner films underlies the presence of strong electrostatic effects.