  Several patterns have been discovered in how some spectral characteristics
change during the decaying phase of long (> few s.) GRB pulses. We compare
these observed signatures with those expected from a relativistically expanding
shell. Within the internal shock model and assuming a short cooling time, we
show that the angular dependence in arrival time can explain the general
characteristics of long GRB pulses. This includes the pulse shape, with a fast
rise and a slower decay, ~ (1+t/tau)^2, and the spectral evolution, which can
be described by the hardness-intensity correlation (HIC), with the intensity
being proportional to the square of the hardness. A variation of the relevant
time scales involved (the angular spreading and the dynamic) can explain the
broad, observed dispersion of the HIC index. Reasonable estimates of physical
parameters lead to situations where the HIC relation deviates from a pure power
law; features that are indeed present in the observations. Depending on the
relative values of the rise and decay times of the intrinsic light curve, the
spectral/temporal behavior, as seen by an observer, will produce hard-to-soft
or tracking pulses. The observed spectrum is a superposition of many intrinsic
spectra arriving from different parts of the shell with varying spectral
shifts. Therefore, it will be broader than the emitted spectrum and its
spectral parameters could have complex relations with the intrinsic ones.
Furthermore, we show that the softening of the low-energy power-law index, that
has been observed in some pulses, can be explained by geometric effects and
does not need to be an intrinsic behavior.
