An improperly conducted approach briefing contributed to a descent below the charted altitude between fixes on an LDA/DME approach to FYV.

Narrative

We were cleared direct to GERTE and 4;500 FT; and cleared for the LDA/DME 34 approach to FYV. Both myself and my Co-pilot had reviewed the approach procedure. I told him to keep me advised as to what altitudes we could descend to on the approach and when we could do so. We turned inbound at GERTE and descended to 4;000 FT. I asked my Co-pilot what altitude we could go down to next; and he responded that we were clear to 3;000 FT. This was outside of AWEMO. As I was descending; we were going through 3;600 FT when he said STOP!!! Just about that time; Razorback Approach told us that they had a low altitude alert on us. I immediately applied power and climbed back to 4;000 FT. From this point we continued the approach with a normal approach and landing. Razorback Approach asked us what had happened while we were still airborne and my Co-pilot responded by saying that our FMS had 'burped'. This is not what happened at all; my Co-pilot misread the approach and admitted it once we were on the ground. He had placed the crossing altitudes into the FMS; but forgot that the VNAV would not work with an LDA approach. This was a very eye opening experience for both of us; and made both of us realize the importance of briefing the approach prior to initiating the approach; other than just reviewing the approach individually.