COMMUTER LTT CROSSES HOLD LINE AND ALMOST ENTERS ACTIVE RWY.

Narrative

ON TAXI-OUT; I WAS RUNNING THE 'TAXI' AND 'BEFORE TKOF' CHKLISTS. SOME OF WHICH REQUIRED CHALLENGE AND RESPONSE; AND OTHERS JUST TO VERBALIZE WITH A RESPONSE. THE CAPT DID NOT HAVE A HEADSET AND WAS USING JUST ONE EARPIECE. OUR INSTRUCTIONS FROM THE GND CTLR WERE TO 'TAXI TO RWY 21 VIA BRAVO AND BRAVO 6 AND YOU WILL BACK TAXI ON RWY 21.' THIS WAS DUE TO ANOTHER ACFT THAT WAS BLOCKING ACCESS TO RWY 21 ON BRAVO AS HE WAS WAITING FOR A FLOW CTL CLRNC. I UNDERSTOOD THIS AS NOT A CLRNC TO TAXI ON TO THE ACTIVE RWY. AS I CONTINUED TO RUN THE CHKLISTS; I WAS DISTRACTED BY THE FACT THAT THE CAPT WAS NOT WEARING A HEADSET WITH A MIKE. I WAS WEARING ONE WITH THE L EAR UNCOVERED. THIS MADE COM EXTREMELY DIFFICULT AND WHEN I VERIFIED THE INSTRUCTIONS WITH HIM; HE NODDED YES. MY CONCERNS ABOUT THIS LACK OF A HEADSET AND DIFFICULT COM WERE VOCALIZED TO THE CAPT PRIOR TO HIS INCIDENT ON THE TRIP. WHILE COMPLETING THE CHKS; I LOOKED OUT AND NOTICED THE CAPT WAS XING THE HOLD SHORT LINE ON B6. I IMMEDIATELY TOLD HIM TO 'STOP; WE MUST CALL TWR FIRST!' HE STOPPED; AND STATED THAT NO WE DID NOT NEED TO CALL THEM. AT THE NEXT MOMENT; GND CALLED AND INSTRUCTED US TO DO JUST THAT. I CONTACTED THEM AND THEY THEN CLRED US TO 'BACK TAXI ON RWY 22 AND YOU'RE CLRED FOR TKOF WHEN YOU'RE READY.' THIS PROB AROSE BECAUSE: CAPT WAS USING AN UNAPPROVED HEADSET -- THUS -- COMMUNICATION IN THE COCKPIT WAS DIFFICULT; VAGUE AND UNCLR VERBIAGE USED BY THE GND CTLR. HUMAN FACTORS TO BE CONSIDERED. CAPT HAD DISPLAYED UNSOUND JUDGEMENT SEVERAL TIMES PRIOR DURING THIS TRIP. THIS CAUSED MYSELF TO BECOME EXTRA VIGILANT OF THE CAPT'S DECISIONS AND PHYSICAL ACTIONS. MORE ACCURATELY; I HAD BECOME HYPERVIGILANT. I WAS PAYING EXTRA ATTN TO MAKE SURE THE PRE TKOF CHKS WERE ALL COMPLETED PROPERLY; PARTICULARLY THE ITEMS HE WAS RESPONSIBLE FOR. THIS ENVIRONMENT WAS RIPE WITH POTENTIAL DISTRS WHICH THIS INCIDENT REVEALS.