Despite the relevance to environmental, biological and industrial processes, the motion of a colloidal particle close to a fluid interface and the way it interacts with the water surface are still largely elusive and intriguing physical phenomena. We explored the system of individual colloidal particles close to an air-water interface by recording highly resolved 3D trajectories through a dual wave reflection interference setup (DW-RIM). Contrary to other established experimental techniques, DW-RIM allows accurate measurements of the absolute particle-interface distance and thus does not require any calibration or assumption to know the location of the interface. The particle-interface system shows two different potential energy landscapes resulting to two different equilibrium particle-interface distances. The larger one can be fairly explained by Van der Waals and electrostatic interactions combined with gravity. The shorter one highlights the existence of an unexpected additional attractive interaction. Surface heterogeneities as possible origin of such an interaction are considered and discussed. Concerning the friction the particle undergoes, the air-water interface peculiarly acts as a slip boundary for the particle motion parallel to the interface and as a no-slip boundary for the perpendicular motion. This experimental result is rationalized considering recent models based on surface incompressibility.