  The problem of quantum scalar field evolution after an instantaneous local
perturbation (quench) is considered. A new approach to descriptions of a quench
from an arbitrary initial state is developed in the framework of the Keldysh
technique. This approach does not require the procedure of the analytical
continuation, which can be ambiguous in some cases. The evolution of the energy
density after local quench is calculated for a simple case, and its dependence
on the interaction region width and the initial conditions is analysed.
