  We present multicolor optical observations of the nearby (z=0.1685) GRB
030329 obtained with the same instrumentation over a time period of 6 hours for
a total of an unprecedented 475 quasi-simultaneous BVR observations. The
achromatic steepening in the optical, which occurs at t~0.7 days, provides
evidence for a dynamic transition of the source, and can be most readily
explained by models in which the GRB ejecta are collimated into a jet. Since
the current state-of-the-art modeling of GRB jets is still flawed with
uncertainties, we use these data to critically assess some classes of models
that have been proposed in the literature. The data, especially the smooth
decline rate seen in the optical afterglow, are consistent with a model in
which GRB 030329 was a homogeneous, sharp-edged jet, viewed near its edge
interacting with a uniform external medium, or viewed near its symmetry axis
with a stratified wind-like external environment. The lack of short timescale
fluctuations in the optical afterglow flux down to the 0.5 per cent level puts
stringent constraints on possible small scale angular inhomogeneities within
the jet or fluctuations in the external density.
