ACR MDT TAXIED ON CLOSED PORTION OF RWY.

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WE WERE FLYING FROM ORD TO SPI. THE CAPT IS VERY VERY LAX ABOUT PROCS AND FARS. I AM VERY BY THE BOOK. THIS HAS CAUSED A FEW CONFLICTS DURING OUR TIME TOGETHER. RWY 22 AT SPI HAS A THRESHOLD DISPLACED 800 FT. ATIS SAID 800 NE FT OF RWY 22 IS CLOSED. THE RWY IS COMPLETELY UNMARKED ON THE CLOSED PORTION. WE WERE RUNNING WAY BEHIND DUE TO A MECHANICAL AT ORD. THE CAPT WAS IN BIG HURRY. I CALLED OUR COMPANY ON COM #2 AND THE CAPT CALLED FOR TAXI ON COM #1 (UNKNOWN TO ME). WHEN I FOUND OUT; I QUERIED HIM AS TO THE TAXI INSTRUCTIONS. HE SAID BACK TAXI ON RWY 22 (MOST TXWYS ARE CLOSED). HE WAS TAXIING FAST AND IT WAS A SHORT TAXI. DURING THE TAXI CHKS; I CALLED TWR TO VERIFY WE WERE CLRED ONTO RWY 22 (HE WAS JUST GOING TO TAXI ON TO THE RWY). IN THE MIDDLE OF THE TAXI CHKS; THE CAPT CALLED FOR THE BEFORE TKOF CHKS. I COULD NOT TELL WHERE THE CLOSED PORTION OF THE RWY BEGAN. UNFORTUNATELY; WITH RUNNING 2 CHKLISTS AND THE FLT ATTENDANTS CALLING THE COCKPIT. I LOOKED UP AND WE WERE ALREADY ON THE CLOSED PORTION. WHERE WE SCREWED UP IS: SHOULD HAVE TAKEN OUR TIME IN A CONFUSING SIT. I SHOULD HAVE SUGGESTED THIS. WE SHOULD HAVE COMPLETED EITHER THE TAXI CHKS OR ALL CHKS BEFORE TAKING THE RWY. VERY POOR COCKPIT RESOURCE MGMNT ON BOTH OF OUR PARTS.