DHC-8 CAPT FORGOT TO HOLD SHORT PER TAXI INSTRUCTION AND WAS SUBSEQUENTLY CLRED ONTO RWY FOR BACK-TAXI AND TKOF.

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AT XC30; WE RECEIVED CLRNC TO TAXI FROM OUR RAMP AT REDMOND; OR (RDM) TO RWY 4 AND HOLD SHORT ON TXWY H. AS I BEGAN TO TAXI; ANOTHER ACR ACFT (A BRASILIA) WAS ALSO WAITING TO TAXI WITH ITS ENGS RUNNING AND THEIR MARSHALER STANDING IN FRONT OF THE ACFT. AS WE STARTED TO TAXI; I WAS PAYING SPECIAL ATTN TO THE ACR'S MARSHALER; TO MAKE SURE WE WERE CLR OF THEM BECAUSE HER BACK WAS TURNED TO US AS WE WERE BEGINNING TO MOVE. THE CLRNC WE RECEIVED WAS TO TAXI TO RWY 4 AND HOLD SHORT AT TXWY H. WE WOULD THEN RECEIVE OUR SQUAWK. BUT AS I WAS DISTR BY THE OTHER MARSHALER; I THOUGHT THAT I HEARD THAT WE COULD GO ONTO RWY 4 AND BACK-TAXI AND HOLD. IF I'M UNSURE OF SOMETHING; I ASK FOR CLARIFICATION. BUT IN MY MIND; I WAS SURE THAT WAS WHAT I WAS SUPPOSED TO DO. MY FO HAD JUST FINISHED WITH THE BEFORE TKOF CHKLIST JUST AS I HAD CROSSED THE HOLD LINE. THE FO SAID TO WAIT. I THEN STOPPED; BUT I WAS IN BTWN THE HOLD LINE AND THE RWY. AT THIS TIME; THE TWR SAW WHAT HAPPENED AND IMMEDIATELY CLRED US ONTO THE RWY AND TO BACK-TAXI INTO POS AND HOLD. AS WE BACK-TAXIED; WE THEN RECEIVED OUR XPONDER CODE AND WE WERE CLRED FOR TKOF. FORTUNATELY; THERE WAS NO OTHER TFC ON THE RWY OR IN THE AIR BECAUSE IT WAS SO EARLY IN THE MORNING. AT LEAST I WAS LUCKY IN THE FACT THAT I DIDN'T ACTUALLY GO ONTO THE RWY ITSELF. I THINK PART OF THE REASON THAT I MESSED UP WAS BECAUSE WE HAD GOTTEN UP AT XA30 TO START OUR DAY AND I WAS USED TO THE SWING SHIFT; SO TO SPEAK. OUR SCHEDULES HAVE EARLY SHOWS MIXED WITH LATE SHOWS; SO I MIGHT HAVE BEEN A LITTLE GROGGY WITHOUT REALLY KNOWING IT. ALSO; MY DISTR WITH THE OTHER ACFT AND THE SAFETY OF THEIR MARSHALER DISTR ME WHEN I SHOULD HAVE BEEN PAYING CLOSER ATTN TO THE INSTRUCTIONS FROM THE CTLR; WHICH WOULD BE EASY TO DO BECAUSE OUR MARSHALER WAS GUIDING US AND THERE IS ENOUGH CLRNC BTWN US AND THE OTHER ACFT'S MARSHALER.