  A general method to construct free quantum fields for massive particles of
arbitrary definite spin in a canonical Hamiltonian framework is presented. The
main idea of the method is as follows: a multicomponent Klein-Gordon field that
satisfies canonical (anti)commutation relations and serves as an auxiliary
higher spin field is introduced, and the physical higher spin field is obtained
by acting on the auxiliary field by a suitable differential operator. This
allows the calculation of the (anti)commutation relations, the Green functions
and the Feynman propagators of the higher spin fields. In addition, canonical
equations of motions, which are expressed in terms of the auxiliary variables,
can be obtained also in the presence of interactions, if the interaction
Hamiltonian operator is known. The fields considered transform according to the
(n/2,m/2) + (m/2,n/2) and (n/2,m/2) representations of the Lorentz group.
