We consider some remarkable properties of the Dirac equation for free field related to the connections of its (regular or singular) solutions with those of the Klein-Gordon equation.The latter play the role of potentials for the corresponding Dirac ``field strength'', and the Dirac equation is form invariant under special gauge transformations of the ``potentials''. Moreover, any solution to Dirac equation can be obtained from only a pair of the Klein-Gordon potentials. Under transformations of space-time coordinates the Klein-Gordon potentials can be considered as scalars while associated Dirac fields behave according to a nonlinear representation of Lorentz group, only in a particular case reducing to the canonical bispinor one. Finally, we present some singular solutions to the Dirac and Klein-Gordon equations, one of which possess a finite conserved charge and can be treated as a ``localized de Broglie wave''.