The ability of colloidal particles to act as emulsion stabilizers was first observed by Pickering [1], who noted that stability arose from the strong attachment of particles to the two-phase interface. Due to the growing interest in the aggregation and rheology of these two-dimensional suspensions, we study the far-field and contact interactions between colloidal particles at a decane-water interface using time-shared optical tweezers. A long-range repulsive force is observed with a power-law dependence on particle separation that is in agreement with dipolar interactions mediated by the fractional dissociation of ionizable surface groups [2]. Furthermore, we find that anionic surfactants added to the aqueous sub-phase result in time-dependent interactions. Finally, the particle interactions are highly anisotropic and exhibit interesting multi-body effects. These experiments provide new insight into the aggregation kinetics, structure and mechanical response of two-dimensional colloidal gels. [1] S. U. Pickering, J. Chem. Soc. 91, 2001 (1907). [2] R. Aveyard et al., Phys. Rev. Lett., 88:246102 (2002).