  The modification of the effect of interactions of a particle as a function of
its pre- and postselected states is analyzed theoretically and experimentally.
The universality property of this modification in the case of local
interactions of a spatially pre- and postselected particle has been found. It
allowed to define an operational approach for characterization of the presence
of a quantum particle in a particular place: the way it modifies the effect of
local interactions. The experiment demonstrating this universality property
provides an efficient interferometric alignment method, in which the beam on a
single detector throughout one phase scan yields all misalignment parameters.
