  We study the evolution of the non-equilibrium quantum fields from a highly
excited initial state in two approaches: the standard Keldysh-Schwinger diagram
technique and the semiclassical expansion. We demonstrate explicitly that these
two approaches coincide if the coupling constant $g$ and the Plank constant
$\hbar$ are small simultaneously. Also, we discuss loop diagrams of the
perturbative approach, which are summed up by the leading order term of the
semiclassical expansion. As an example, we consider shear viscosity for the
scalar field theory at the leading semiclassical order. We introduce the new
technique that unifies both semiclassical and diagrammatic approaches and open
the possibility to perform the resummation of the semiclassical contributions.

High Energy Physics - Phenomenology;Statistical Mechanics;High Energy Physics - Theory;Nuclear Theory