CAPT OF ACR MLG ASKED A DEAD HDG CAPT TO REPLACE HIM FOR THE FLT BEFORE TKOF FROM IAH. THE CHANGE TOOK PLACE AND AT EWR THE REPLACED CAPT WAS GIVEN A DRUG TEST.

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(IAH) WHILE WAITING FOR DEP IN PAX CABIN AS A PAX COMMUTING TO WORK (EWR). I WAS APCHED BY THE FLT ATTENDANT AND ASKED MY BASE AND EQUIP. THE FLT ATTENDANT WENT TO THE COCKPIT AND RETURNED TO ASK IF I WOULD FLY THE TRIP TO EWR; AND TO SPEAK WITH THE CAPT. I WENT FORWARD AND ASKED THE CAPT WHAT HE NEEDED. HE ASKED IF I WOULD FLY THE TRIP TO EWR. I AGREED AND ASKED ABOUT CALLING SCHEDULING. HE MENTIONED IT WAS 2 MIN UNTIL DEP TIME AND SAID WE WOULD MAKE THE FLT LATE AND WOULD APPRECIATE IT IF I JUST TOOK THE FLT. I TOLD HIM OF MY TRIP LATER ON IN THE DAY AND HE SAID 'IF WE JUST SWAP; THEN THERE WOULD BE NO PROBLEM THERE EITHER.' I THEN AGREED AND TOOK HIS SEAT AND FLEW TO EWR. ARR AT EWR THE FLT WAS MET BY SOMEONE FROM THE EWR CHIEF PLT'S OFFICE TO ADMINISTER RANDOM DRUG TEST FOR THE CAPT I REPLACED. I THEN WENT TO THE OFFICE AND WAS REMOVED FROM MY TRIP LATER IN THE DAY. THE DECISION I MADE; IN A RELATIVELY SHORT TIME; HAD A POTENTIAL FOR FAR REACHING PROBLEMS. SOMETIMES IT IS EASY TO BE LED INTO A SITUATION THAT SEEMS INNOCENT ON THE SURFACE BECAUSE THE PRACTICE MANY YRS AGO WAS ACCEPTABLE BUT IN TODAY'S ENVIRONMENT IS FAR MORE COMPLICATED. IN ADDITION TO THE FORGOING I CHKED THE WX; FLT PLAN AND FUEL BUT FAILED TO SIGN THE RELEASE.