  We explore a subtle and fundamental nature of spin-orbit interaction (SOI) in
a general paraxial beam-field by mathematically characterizing the orbital
angular momentum (OAM) flux densities of the spin-polarized component fields
via Barnett's flux determination formalism. We show the application of our
formalism by considering the special case of a Brewster-reflected paraxial
beam, for which we explore specific details such as non-canonical vortex phase
structures and single-photon interpretation of the field function. The phases
of the spin-component fields are interpreted as geometric phases; and we devise
an experimental method of direct measurement of these phases by transferring
the spatial phase information to the polarization domain -- thus demonstrating
a classical analog von Neumann measurement. Our complete mathematical
characterization of SOI and its relevance to the direct measurement of
geometric phase can find potential application in future methods exploring OAM
and geometric phase characteristics in other general optical systems.
