B737-700 CREW HAD A RWY INCURSION AT SNA THAT REQUIRED A PA28 PLT TO GO AROUND.

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WHILE HOLDING SHORT OF TXWY L ON TXWY A; TWR INSTRUCTED US TO TAXI TO RWY 19R; HOLD SHORT RWY 19L. THE TAXI CLRNC WAS READ BACK AND THE CAPT STARTED THE TAXI. SHORTLY AFTER WE STARTED THE TAXI I WAS LOOKING IN THE COCKPIT TO DO SOME LAST MIN FMC INPUTS. I LOOKED UP TO SEE THE CAPT GETTING READY TO CROSS RWY 19L. I SAID 'WE NEED TO HOLD SHORT OF RWY 19L;' BUT IN RETROSPECT I SHOULD HAVE BEEN MORE ASSERTIVE IN MY ACTIONS. THE CAPT CROSSED RWY 19L. THE TWR REALIZED OUR SIT AND INSTRUCTED US TO THEN HOLD SHORT OF RWY 19R. IT HAD BEEN A LONG DAY ALREADY WITH A 3 1/2 HR DELAY AND AN AIRBORNE MECHANICAL WHICH RESULTED WITH RETURN TO ZZZ AFTER TKOF. CONTRIBUTING FACTORS WERE PROBABLY FATIGUE ALTHOUGH WE DIDN'T REALIZE IT. SUPPLEMENTAL INFO FROM ACN 621986: I LOOKED TO THE R AND SAW AN ACFT ON FINAL FOR THE RWY AND NOTHING FOR THE L. AS I CROSSED THE LIMIT LINE; I HEARD THE FO SAY SOMETHING (HESITANTLY) THAT DIDN'T REGISTER AT FIRST. THEN IT STRUCK ME THAT HE HAD SAID 'HOLD SHORT.' I STARTED TO STOP AND LOOKED OUT THE R SIDE WINDOW UP HIGH AND SAW A CHEROKEE STARTING A GAR. MY FO WAS ON HIS FIRST REVENUE FLT AFTER HIS IOE TRAINING AND WAS DOING A FINE JOB; BUT REQUIRED SOME EXTRA ATTN FROM ME DURING THE PREFLT PLANNING AND PREPARATION FOR EACH LEG. HE WAS ALSO A LITTLE UNSURE OF HIMSELF AND; THEREFORE; DID NOT ADVOCATE WHAT HE KNEW WAS RIGHT UNTIL IT WAS TOO LATE. I TALKED WITH MY FO ABOUT THE NEED TO ADVOCATE INFO MORE FORCEFULLY AND CLRLY WHEN IT IS NECESSARY; EVEN TO THE POINT OF STEPPING ON THE BRAKES WHEN THE AIRPLANE SHOULD BE STOPPED AND THE CAPT ISN'T STOPPING IT. I WILL BE VERBALLY RECONFIRMING CLRNCS TO CROSS ALL RWYS OR TO HOLD SHORT WITH MY FO BEFORE XING ANY LINES. THIS WILL BE DIFFICULT BECAUSE IT IS DURING A VERY BUSY TIME FOR BOTH PLTS; BUT I BELIEVE IT IS A CRITICAL STEP IN PREVENTING THIS KIND OF INCURSION FROM HAPPENING AGAIN.