  The complex time WKB approximation is an effective tool in studying particle
production in curved spacetime. We use it in this work to understand the
formation of classical condensate in expanding de Sitter spacetime. The CWKB
leads to the emergence of thermal spectrum that depends crucially on horizons
(as in de Sitter spacetime) or observer dependent horizons (as in Rindler
spacetime). A connection is sought between the horizon and the formation of
classical condensate. We concentrate on de Sitter spacetime and study the
cosmological perturbation of $k=0$ mode with various values of $m/H_0$. We find
that for a minimally coupled free scalar field for $m^2/H_0^2<2$, the one-mode
occupation number grows more than unity soon after the physical wavelength of
the mode crosses the Hubble radius and soon after diverges as $N(t)\sim
O(1)[\lambda_{phys}(t)/{H_0^{-1}}]^{2\sqrt{\nu^2-1/4}}$, where $\nu\equiv (9/4
-m^2/{H_0^2})^{1/2}$. The results substantiates the previous works in this
direction. We also find the correct oscillation and behaviour of $N(z)$ at
small $z$ from a single expression using CWKB approximation for various values
of $m/H_0$. We also discuss decoherence in relation to the formation of
classical condensate. We also find that the squeezed state formalism and CWKB
method give identical results.
