A fatigued Captain on a long flight to ZGGG lost situational awareness and descended early when the autopilot failed to capture the localizer and had to be disconnected. A GPWS altitude callout alerted the First Officer to the error.

Narrative

I was the pilot not flying during approach into Guangzhou; China. We were cleared to 3;000 FT and cleared for the ILS. When the localizer captured; the plane initially turned the wrong direction. The Captain (Pilot Flying) made a manual heading correction and re-intercepted the ILS. After this; I was heads down performing the landing checklist when I heard a GPWS '1;000' callout. We were actually at about 2;500 FT MSL with an assigned altitude of 3;000 FT MSL. The '1;000' callout was likely due to terrain. I looked up to see that we had descended below the glidepath by about 500 FT. I realized that the Captain had commanded an early descent. I querried him about this and he made a correction to stop the descent. About the same time; ATC called and asked us if we were on the glideslope...they seemed concerned about our altitude. After the flight the Captain explained that he got behind when the jet initially turned the wrong way. That in conjunction with being tired caused him to misinterpret the instruments and gave him the perception that he had to descend to intercept the glide path when in fact we were still below and had not yet intercepted. We discussed the need to always confirm new altitudes. If the Captain had verbalized what he was doing before he did it; that would have helped too. In the future; I will also try to minimize heads down time when performing checklist duties in order to provide a better back up for the pilot flying.