ENG LEFT RUNNING AFTER FLT. DISCOVERED WHEN WALKAROUND WAS COMMENCED AND NEXT CREW ARRIVED.

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ACFT TAXIED TO GATE. APU WAS MEL'ED INOP. R ENG WAS LEFT RUNNING TO PROVIDE ELECTRICAL PWR; BECAUSE GND PWR COULD NOT BE IMMEDIATELY ESTABLISHED. PAX DEPLANED AND GND PWR WAS ESTABLISHED. FO LEFT FLT DECK TO PERFORM WALKAROUND. CAPT MOVED FUEL LEVER OF R ENG TO SHUTOFF POS. CAPT WAS IN A HURRY TO CATCH A DEPARTING FLT HOME AND VACATED THE FLT DECK. CAPT DID NOT NOTICE THAT ENG WAS STILL RUNNING. FLT DECK WAS VACANT FOR SEVERAL MINS UNTIL FO NOTICED ENG WAS RUNNING ON HIS WALKAROUND AND RETURNED TO FLT DECK ABOUT THE SAME TIME AS THE INCOMING CREW ALSO ENTERED THE FLT DECK. FORTUNATELY; BRAKES WERE SET AND ACFT WAS CHOCKED SO THE ACFT DID NOT MOVE! OBVIOUSLY; A DANGEROUS SIT! IN LOOKING BACK; I BELIEVE THIS SIT WAS CAUSED BY SEVERAL FACTORS. FIRST; AN UNUSUAL OR OUT-OF-THE-ORDINARY SIT (NO APU OR GND PWR). SECOND; HURRY TO LEAVE ACFT TO GET HOME. THIRD; FAILURE TO COMPLETE THE SHUTDOWN CHKLIST CAREFULLY. CONTRIBUTING TO THIS INCIDENT IS THE DESIGN OF THE PWR/THRUST LEVER IN THE CRJ. THE SHUTOFF POS IS A VERY SMALL MOVEMENT OF THE LEVER ONCE A RELEASE HAS BEEN LIFTED. IT IS POSSIBLE TO LIFT THE RELEASE AND MOVE THE LEVER; YET NOT MOVE IT FAR ENOUGH AGAINST SOME RESISTANCE TO SHUT OFF ENG. ALSO; FO; IN HIS WILLINGNESS TO HELP; DID NOT PREPARE ME TO NOTE ALL THE CLUES THAT ENG WAS NOT SHUT DOWN. I ALWAYS GO SLOW NOW! AND PERFORM CHKLISTS DELIBERATELY AND DOUBLECHK ENG GAUGES TO BE SURE OF ENG SHUTDOWN.