  The prompt gamma-ray emission in gamma-ray bursts is believed to be produced
by internal shocks within a relativistic unsteady outflow. The recent detection
of prompt optical emission accompanying the prompt gamma-ray emission appears
to be inconsistent with this model since the out flowing plasma is expected to
be highly optically thick to optical photons. We show here that fluctuations in
flow properties on short, ~ 1 ms, time scale, which drive the gamma-ray
producing collisions at small radii, are expected to lead to "residual"
collisions at much larger radii, where the optical depth to optical photons is
low. The late residual collisions naturally account for the relatively bright
optical emission. The apparent simultaneity of gamma-ray and optical emission
is due to the highly relativistic speed with which the plasma expands. Residual
collisions may also account for the X-ray emission during the early "steep
decline" phase, where the radius is inferred to be larger than the gamma-ray
emission radius. Finally, we point out that inverse-Compton emission from
residual collisions at large radii is expected to contribute significantly to
the emission at high energy, and may therefore "smear" the pair production
spectral cut-off.
