  GRB 990123 was a long, complex gamma-ray burst accompanied by an extremely
bright optical flash. We find different constraints on the bulk Lorentz of this
burst to be consistent with the speculation that the optical light is emission
from the reverse shock component of the external shock. Motivated by this
currently favoured idea, we compute the prompt reverse shock emission to be
expected for bursts in which multi-wavelength observations allow the physical
parameters to be constrained. We find that for reasonable assumptions about the
velocity of source expansion, a strong optical flash of approximately m_V=9 was
expected from the reverse shocks, which were usually found to be mildly
relativistic. The best observational prospects for detecting these prompt
flashes are highlighted, along with the possible reasons for the absence of
optical prompt detections in ongoing observations.
