A DH8B CREW STARTED THEIR TKOF ROLL AT FLG WITHOUT A CLRNC.

Narrative

I WAS THE FO AND PNF ON A PART 121 SCHEDULED FLT FOR XYZ AIRLINES FROM FLAGSTAFF PULMAN ARPT; FLAGSTAFF; AZ TO PHOENIX SKY HARBOR ARPT; PHOENIX; AZ. AFTER HOLDING SHORT OF RWY 21; OUR FLT WAS CLRED INTO POS. A CONVERSATION THAT HAD BEGUN WHILE HOLDING SHORT OF THE RWY CONTINUED FOR A FEW MOMENTS AFTER I READ BACK THE CLRNC TO TAXI INTO POS ON THE RWY. WE THEN COMPLETED FINAL CHKLIST ITEMS BEFORE TKOF. I DO NOT RECALL IF THE CAPT CONFIRMED THE HOLD IN POS CLRNC TO ME; AS WAS OUR USUAL CUSTOM. THE CAPT BEGAN TO PWR UP THE ACFT ENGS SLOWLY WITH BRAKES ON; STILL HOLDING IN POS; WHILE I MONITORED THE ENG INSTS. THE NEXT THING I HEARD WAS THE CAPT STATE 'SET TKOF PWR' WHILE RELEASING THE BRAKES AND LETTING THE ACFT BEGIN TO ROLL. THEN TWR SAID 'XYZ; I CLRED YOU INTO POS; CONTINUE; YOU ARE CLRED FOR TKOF.' WE COMPLETED THE TKOF ROLL AND PROCEEDED WITHOUT FURTHER COMMENT OR INCIDENT. THERE WAS NO OTHER TFC AT THE ARPT AND NO FURTHER ACTION WAS REQUIRED. I FEEL THAT THE DISTR OF THE LAST BIT OF CONVERSATION WE HAD AS WE BEGAN OUR TAXI ONTO THE RWY AND BEFORE COMPLETION OF OUR CHKLIST WAS ENOUGH TO MAKE US HAVE THIS OCCURRENCE. I DIDN'T REMEMBER IF I GOT CONFIRMATION FROM THE CAPT ON THE CLRNC; AND I DIDN'T CATCH THE MISTAKE WHEN HE TOLD ME TO SET THE TKOF PWR. SOME CAPT'S CHOOSE NOT TO TURN ON ALL OF THE ACFT LIGHTING UNTIL TKOF CLRNC IS CONFIRMED; AS A REMINDER. IN HINDSIGHT; I FEEL LIKE I SHOULD HAVE NOT PARTICIPATED IN THE CONVERSATION'S CONTINUANCE UNTIL REACHING A MORE APPROPRIATE PHASE OF FLT.