EMB145 FLT CREW HAD A RWY INCURSION AT EWR.

Narrative

CAPT AND I CLRED RWY 22L AND RECEIVED AN INSTRUCTION TO TAXI PARALLEL THE RWY; THEN HOLD SHORT OF RWY 22R AT A SPECIFIC TXWY. I READ BACK THE INSTRUCTION THEN VERBALLY CONFIRMED THE INSTRUCTION WITH MY CAPT. I THEN HAD MY HEAD INSIDE THE COCKPIT TO COMPLETE MY AFTER LNDG FLOW AND CHKLIST. WHEN I LOOKED OUT AGAIN WE HAD JUST CROSSED THE HOLD SHORT LINE TO RWY 22R. I REMINDED THE CAPT THAT WE WERE TO HOLD SHORT AND HE STOPPED THE ACFT MAYBE 10 FT BEYOND THE HOLD SHORT LINE. HERE'S WHERE THINGS GOT VERY EDUCATIONAL. AFTER STOPPING THE ACFT; THE CAPT ELECTED TO PUT THE ACFT INTO REVERSE THRUST AND TRY TO BACK UP. I IMMEDIATELY TOOK OVER THE BRAKES AND SAID THAT WE SHOULD JUST WAIT HERE AND NOT MAKE THE SITUATION ANY WORSE. HE GOT THE REVERSERS BACK IN AND WE WAITED FOR OUR CLRNC TO CROSS. I THINK IT'S A BAD IDEA TO TRY TO REVERSE AN ACFT WITHOUT GUIDANCE FROM GND PERSONNEL OR A PROC TO FOLLOW. I DON'T BELIEVE THAT THE TWR SAW THAT WE HAD CROSSED THE HOLD SHORT LINE AS THEY GAVE 2 CLRNCS FOR TKOF WHILE WE WERE IN THAT POS. THE IMPORTANT LESSON OUT OF THIS IS TO MAINTAIN VIGILANCE OUTSIDE THE COCKPIT NO MATTER WHAT YOU ARE DOING. IF I'D KEPT MY HEAD OUT THE WINDOW THIS MAY NOT HAVE TAKEN PLACE.