Intermediate-long-slit spectroscopy of the protoplanetary-nebula candidate CRL 618, obtained using an IPCS on the 2.5-m telescope at the Observatorio Roque de los Muchachos, are reported. The data are presented in plots of forbidden N II (6548 + 6584 A) vs H-alpha and compared with the predictions of theoretical models. Best agreement is found with the model of Canto (1978 and 1980), in which the outflow from the central B star is supersonic with respect to the surrounding medium and two shock waves develop, one propagating into the stellar wind and one moving into the envelope of the primary. In this scenario the expansion velocities of the molecular and ionized gas are 20 and 80 km/sec, respectively; the mass-loss rate is a few times 10 to the -6th solar mass/yr; the velocity of the fast wind is about 140 km/sec; the shocked-wind density is 100-1000/cu cm; the shocked-gas density in the principal outflow envelope is 1,000-10,000/cu cm; and the shock velocity within the envelope is about 50 km/sec.