  The Bogoliubov transformation in cosmological particle production can be
explained by the Stokes phenomena of the corresponding ordinary differential
equation. The calculation becomes very simple as far as the solution is
described by a special function. Otherwise, the calculation requires more
tactics, where the Exact WKB (EWKB) may be a powerful tool. Using the EWKB, we
discuss cosmological particle production focusing on the effect of more general
interaction and classical scattering. The classical scattering appears when the
corresponding scattering problem of the Schr\"odinger equation develops
classical turning points on the trajectory. The higher process of fermionic
preheating is also discussed using the Landau-Zener model.
