  Prompt optical emission from the $\gamma$-ray burst of GRB 041219A has been
reported by Vestrand et al. There was a fast rise of optical emission
simultaneous with the dominant $\gamma$-ray pulse, and a tight correlation with
the prompt $\gamma$-ray emission has been displayed. These indicate that the
prompt optical emission and $\gamma$-ray emission would naturally have a common
origin. We propose that this optical component can be modeled by considering
the Comptonization of $\gamma$-ray photons by an electron cloud. As a result of
this mechanism, the arrival time of the optical photons is delayed compared
with that of the $\gamma$-rays. We restrict that the lag time to be shorter
than 10 s, within which the prompt optical emission is considered to vary
simultaneously with the prompt $\gamma$-ray emission. Taking the observations
of GRB 041219A into account, we derive the number density of the surrounding
electron cloud required for Comptonization. The redshift of GRB 041219A is
predicted to be $z\lesssim 0.073$ as well.
