DEADHEADING CAPT REPLACED THE CAPT SCHEDULED TO FLY WITHOUT AUTH FROM ACR.

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I FLEW TRIP FOR ANOTHER CAPT. DEPARTING PHX AT XA10 LCL ARRIVING IN DEN AT XG58 LCL. I AM CURRENTLY A RATED CAPT ON THE MLG ACFT AND ALL MY PHYSICAL; PROFICIENCY CHKS AND RATINGS ARE CURRENT. I HAD IN MY POSSESSION ALL REQUIRED MAPS; CHARTS AND REGS. I READ THE DISPATCH RELEASE AND CONCURRED THAT THE FLT COULD BE FLOWN SAFELY AS DISPATCHED. THE F/A'S WERE INFORMED THAT I WAS REPLACING THIS OTHER CAPT. FLT DISPATCH WAS ALSO INFORMED ON THE MON IN OCTOBER I FLEW A TOTAL OF 6 HOURS; 55 MINUTES IN THE MLG AND WAS ON DUTY FOR 13 HOURS AND 15 MINUTES. NO PROBS WERE ENCOUNTERED ON THE FLT AND NONE WERE ENCOUNTERED ON ANY SUBSEQUENT FLTS THAT DAY. SUPPLEMENTAL INFO FROM ACN 96449. THE CAPT WHICH WAS ASSIGNED TO THE FLT ASKED THE CONNECTING CAPT IF HE WOULD BE WILLING TO TAKE THE FLT AND ALLOW THE ASSIGNED CAPT TO REMAIN IN PHX WHERE HE LIVED. ALTHOUGH THIS TYPE OF ACTION IS NOT PERMITTED BY THE AIRLINE; THE PREVIOUS AIRLINE WITH WHICH (BEFORE MERGER) BOTH CAPTS HAD FLOWN; ALLOWED SUCH PRACTICES. BETTER JUDGEMENT BY BOTH CAPTS WOULD ELIMINATE THIS PROB. CALLBACK CONVERSATION WITH RPTR REVEALED FOLLOWING INFO. CAPT ADMITTED THAT HE DID NOT CHANGE THE SIGNATURE ON THE DISPATCH RELEASE. HE FEARS THAT HIS COMPANY IS 'OUT TO GET HIM' BECAUSE HE WALKED THE PICKET LINE DURING A PAST WORK STOPPAGE.