ACR RUNS UP ENG FOR ANTI-ICE PROC WHILE WAITING FOR TKOF NOT AWARE OF SMALL ACFT JUST BEHIND. ACFT BEHIND CALLS OUT POS AND ACR STOPS BEFORE ANY DAMAGE IS DONE.

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AFTER DEICING; WE COMPLETED OUR CHKLISTS AND PREPARED FOR TKOF. WE STILL HAD TO DO AN ENG RUNUP AS PER OUR FLT MANUAL TO SHED ANY ICE BUILDUP ACCUMULATED ON TAXI. WHILE #2 FOR TKOF; WE OBSERVED AN EMER ACFT ON FINAL (TALKING WITH TWR) FOR OUR RWY. THE PRECEDING ACFT AWAITING TKOF WAS ISSUED AN IMMEDIATE TKOF CLRNC AND WE DEDUCED FROM THE CONVERSATION THAT WE WOULD BE CLRED FOR AN IMMEDIATE TKOF ALSO. I WANTED TO DO THE RUNUP WHEN IN POS (15 SECONDS); BUT DIDN'T WANT TO DELAY THE EMER ACFT. NOW #1; WE TURNED 90 DEGS TO RWY AND HELD SHORT. I LOOKED BEHIND AND TO MY L AND OBSERVED THE FOLLOWING B737 HAD HELD PARALLEL TO THE RWY AND WELL CLR. BECAUSE I/WE WERE SURE(!) THERE WAS NOBODY BEHIND US; WE STARTED A SLOW PWR INCREASE TO DO OUR RUNUP. MY MISTAKE WAS NOT FOLLOWING MY JUDGEMENT AND GETTING THE AREA BEHIND US CLRED. HOWEVER; ATC WAS VERY BUSY WITH COORDINATING EMER VEHICLES; ETC; AND IT WASN'T FEASIBLE AT THE TIME. ABOUT 1/2 WAY THROUGH; A BEECH 1900D DIRECTLY BEHIND US NOTED HIS POS ON TWR FREQ. WE/I IMMEDIATELY RETARDED PWR AND ASKED IF THEY WERE OK; AND THEY RESPONDED IN THE AFFIRMATIVE. WE'RE STILL NOT SURE HOW HE CAME TO BE BEHIND US -- PROBABLY FROM THE SECOND PARALLEL. LESSON: TRUST YOUR JUDGEMENT AND WHEN IN DOUBT; JUST WAIT! WE FINALLY TOOK OFF AFTER THE EMER ACFT LANDED AND HAD PLENTY OF TIME FOR OUR RUNUP -- ANOTHER GREAT LESSON LEARNED.