AN ACR LTT CREW ATTEMPTED TO TAXI WITH THEIR AIRSTAIR DOOR OPEN.

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AFTER ENG START; THE CHOCKS WERE REMOVED AND; AS THE BEFORE TAXI CHK WAS BEING READ OFF; I STARTED TO MOVE THE AIRPLANE AS BEING DIRECTED BY THE FLAGMAN. AFTER ABOUT 2 FT; HE TOLD US TO STOP. WE THEN NOTICED THAT THE AIRSTAIR DOOR IN BACK WAS STILL OPEN. THE FO THEN GOT UP TO SHUT IT AND WE CONTINUED ON. I USUALLY DON'T CLOSE THE DOOR. I'M USUALLY UP FRONT FIRST DOING THE REQUIRED PAPERWORK. THE FLOW OF NORMAL DUTY ASSIGNMENTS WAS SHIFTED. I DID NOT CATCH THE DOOR LIGHT BEFORE THE ENGS WERE STARTED. THE DOOR LIGHT USED TO BE ON THE BEFORE ENG START CHK; NOW IT'S BEFORE TAXI CHK WHICH I DON'T AGREE WITH. THE BEFORE START DOES HAVE A CAUTION 'LIGHT TEST' ON IT BUT; DUE TO THE BRIGHTNESS OF THE DAY AND GLARE; I DID NOT CATCH THE DOOR CAUTION LIGHT. I FEEL IT WAS A COMBINATION OF EVENTS THAT LED TO THE SIT. MYSELF COMING UP TO THE COCKPIT AND DOING THE PAPERWORK AFTER THE FO WAS ALREADY ON BOARD. THE DOOR NOT BEING ON THE BEFORE START CHKLIST. COMPLACENCY. AFTER FLYING 90-100 HRS EVERY MONTH A CERTAIN FLOW IS ESTABLISHED WHICH AT THAT TIME WAS BROKEN.