COMMUTER LTT ALTDEV OVERSHOT DURING DSCNT.

Narrative

DURING CRUISE FLT OCEAN APCH CLRED US FROM 10000 FT TO 8000 AND DIRECT TO CALVERTON VOR. THE CAPT RESPONDED TO THE RADIO. I SELECTED THE VOR TO CCC; TURNED THE HDG BUG (ACFT WAS ON AUTOPLT) AND SELECTED VERT SPD OF 600 FPM. THE ACFT TURNED AND PROCEEDED DOWN AT 600 FPM. AT THAT POINT THE FLT ATTENDANT ENTERED THE COCKPIT TO DELIVER COFFEE AND LEFT THE COCKPIT. 2 MINS LATER THE CAPT ASKED WHAT ALT WE WERE GOING TO; WHICH PROMOTED ME TO CHK THE ALT; WHICH INDICATED 7400 FT. I TOOK THE AUTOPLT OFF AND CLBED QUICKLY TO 8000 FT. THE CAUSE: I ASSUMED THE CAPT SELECTED THE ALT WHEN RECEIVING THE NEW CLRNC; BUT AS THE PF NEGLECTED TO CHK THE ALT SELECTOR WHEN THE FLT ATTENDANT CAME INTO THE COCKPIT. THE DISTR OF THE FLT ATTENDANT; AT THAT MOMENT; CHANGED MY FLOW OF DUTIES WHICH MADE ME DSND THROUGH MY ASSIGNED ALT OF 8000 FT. EVEN THOUGH THIS MISTAKE DID NOT CAUSE A PROBLEM; IT HAS OPENED MY EYES ONCE AGAIN TO ALWAYS BE FOCUSED ON THE JOB AT HAND AND NOT LET DISTRS INTERRUPT YOUR FLOW OF DUTY.