A COMMUTER CAPT WAS SLOW TO INITIATE DSCNT TO COMPLY WITH CLRNC. A VERBAL EXCHANGE ENSUED BTWN THE FLC AND ATC.

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I WAS ON #2 RADIO PICKING UP SGF ATIS AND CALLING COMPANY AT DEST; SGF. WHEN I RETURNED TO #1 RADIO; ATC CHANGED OUR DSCNT CLRNC. PREVIOUSLY; WE HAD PLT DISCRETION DSCNT FROM 20000 FT TO APPROX 8000 FT. PF HAD LEVELED OFF AT 18000 FT; TO REMAIN ON TOP OF OVERCAST. NOW ATC REQUESTED WE LEAVE 18000 FT; THEN PLT DISCRETION BELOW 15000 FT (APPROX). I READ BACK 'OUT OF 18;' HOWEVER PF DID NOT VACATE 18000 FT RIGHT AWAY. HE WAITED APPROX 80 SECONDS. HE SEEMED TO BE WAITING FOR A LOWERING IN TOPS TO APPEAR; PRESUMABLY TO MINIMIZE TURB. IN MEANTIME; ATC SAID WE WERE 'STILL AT 18000 FT ON RADAR?' WHEN HE SAID THAT CAPT BEGAN DSCNT OUT OF 18000 FT. I; ONCE AGAIN; SAID 'OUT OF 18000 FT.' ATC; IN ANGRY TONE; ASKED OUR ALT. I SAID 'PASSING 17800 FT.' REST OF FLT WAS UNEVENTFUL. ALTHOUGH CAPT SEEMED NERVOUS OVER ATC'S REACTION TO HIS NON- COMPLIANCE. THIS CAPT WAS JUNIOR-MANNED FOR THIS LEG ONLY. I BELIEVE THIS HAD LOWERED HIS MORALE; HENCE PERFORMANCE. NORMALLY; I READ BACK DSCNT CLRNCS BY SAYING 'OUT OF....FOR...ALT;' ASSUMING PF IS FIXING TO COMPLY WITH CLRNC. IN THIS CASE; THERE WAS NO WAY FOR ME TO GUESS THAT HE WAS NOT GOING TO COMPLY RIGHT AWAY! IN FUTURE; I WILL AVOID SAYING 'OUT OF CERTAIN ALT;' UNTIL I WITNESS THIS TAKING PLACE; UNLESS I KNOW THE PF WELL.