  We develop a new method for simultaneous timing and spectral studies of Gamma
Ray Burst (GRB) prompt emission and apply it to make a pulse-wise description
of the prompt emission of GRB 090618, the brightest GRB detected in the Fermi
era. We exploit the large area (and sensitivity) of Swift/BAT and the wide band
width of Fermi/GBM to derive the parameters for a complete spectral and timing
description of the individual pulses of this GRB, based on the various
empirical relations suggested in the literature. We demonstrate that this
empirical model correctly describes the other observed properties of the burst
like the variation of the lag with energy and the pulse width with energy. The
measurements also show an indication of an increase in pulse width as a
function of energy at low energies for some of the pulses, which is naturally
explained as an off-shoot of some particular combination of the model
parameters. We argue that these model parameters, particularly the peak energy
at the beginning of the pulse, are the natural choices to be used for
correlation with luminosity. The implications of these results for the use of
GRBs as standard candles are briefly described.
