The modification of the effect of interactions of a particle as a function of its preselected and postselected states is analyzed theoretically and experimentally, and the universality of this modification in the case of local interactions of a spatially preselected and postselected particle is demonstrated. It allows us to define an operational approach for the characterization of the presence of a quantum particle in a particular place, namely the way it modifies the effect of local weak interactions. The experimental implementation, which entangles the transverse degrees of freedom of photons with their path in a Mach-Zehnder interferometer, also allows us to demonstrate a novel and efficient method of interferometer alignment, where the trajectory of the postselected beam on a single quadrant detector, throughout a single phase scan, yields all the misalignment parameters.