RWY 35R INCURSION AT LOUISVILLE; KY; BY A C550 WITH TRAFFIC CLRED INTO POSITION AT OPPOSITE END OF RWY.

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I WAS FLYING WITH AN UNFAMILIAR CAPT. I ACKNOWLEDGED THE CLRNC TO TAXI TO HOLD SHORT OF RWY 35R. OUR RADIOS AND INTERCOM WERE WORKING FINE. THE CAPT HEARD THE CLRNC; BUT WHETHER HE COMPLETELY HEARD ALL OF IT; IN RETROSPECT; I DON'T KNOW. I WAS LOOKING AT A CHART TRYING TO FIND OUR FIRST FIX. THE CAPT TAXIED PAST THE HOLD SHORT LINES WHICH ARE WELL BACK OF RWY 35R. NO PART OF THE AIRPLANE WAS ON THE RWY. THE CTLR WAS ON THE BALL. I LOOKED UP AND SAID 'THIS IS RWY 35' JUST AS SHE TOLD US TO 'HOLD SHORT; YOU ARE ON THE RWY.' TECHNICALLY I GUESS SHE WAS RIGHT -- WE WERE PAST THE HOLD SHORT LINES. (I DON'T THINK THE WHOLE AIRPLANE WAS -- AS I SAID; NO PART OF THE AIRPLANE WAS ACTUALLY ON THE RWY ITSELF.) AT THE FAR END; ANOTHER AIRPLANE WAS TAXIING INTO POS FOR TKOF AS THIS HAPPENED. THE CTLR TOLD US TO GO AHEAD; HE WOULD HOLD FOR US. I DON'T KNOW WHETHER THE CAPT WOULD HAVE STOPPED WITHOUT ME AND THE CTLR SAYING SOMETHING. I SHOULD NOT HAVE BEEN LOOKING AT A CHART WHILE WE WERE TAXIING TO AN INTXN. EXPERIENCE FLYING WITH PLTS I KNEW BETTER HAD MADE ME COMPLACENT. LOOKING DOWN THE RWY AT THE AIRPLANE NOW HOLDING SHORT FOR US AS WE TAXIED ACROSS THE RWY MADE ME THINK; 'WHAT IF IT HAD BEEN A FEW SECONDS LATER AND THAT ACFT HAD BEEN ON ITS TKOF ROLL AND THE VISIBILITY HAD BEEN LESS?' I NOW MORE FULLY UNDERSTAND THAT TAXIING ON AN ARPT IS A CRITICAL PHASE OF FLT.