FLC OF AN MLG FAILED TO TURN OVER A STAR FIX RESULTING IN ATC INTERVENTION TO BRING THEM BACK ON COURSE.

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DURING THE LATE STAGES OF FLT FROM IAH TO EWR; WE WERE FLYING DIRECT TO THE PXT VOR ON APPROX A 060 DEG HDG. AFTER PXT VOR; WE WERE CLRED TO FLY THE WARRD 4 ARR TO EWR. THE WARRD 4 STAR REQUIRES FLYING A 046 DEG RADIAL OUT OF PXT. JUST PRIOR TO PXT; A FLT ATTENDANT ENTERED THE COCKPIT. THE FO; FLT ATTENDANT; AND MYSELF ENTERED INTO A CONVERSATION. THE DISCUSSION WAS NOT RELATED TO OUR FLT. DURING THE CONVERSATION WE CROSSED THE PXT VOR. I; HOWEVER; MISSED THAT FACT WHILE INVOLVED IN THE DISCUSSION. SHORTLY THEREAFTER; ZDC ADVISED US TO MAKE A COURSE CORRECTION TO REJOIN THE WARRD 4 ARR. MY LACK OF ATTN TO THE TASK AT HAND CAUSED US TO DEVIATE 6 MI OFF THE ARR RTE. WE DEVIATED E OF THE AIRWAY; AND VERY CLOSE; IF NOT INTO RESTR AREA R4008. WE QUICKLY REINTERCEPTED THE 046 DEG RADIAL OFF PXT; AND CONTINUED THE FLT UNEVENTFULLY. IT HAS BECOME VERY CLR TO ME FROM THIS INCIDENT THAT ALTHOUGH WE HAD NOT YET ENTERED A CRITICAL PHASE OF FLT; THAT INCREASED VIGILANCE IS REQUIRED WHEN TRYING TO FLY THE AIRPLANE; NAV A STAR; AND TAKE PART IN A 3-WAY CONVERSATION. I VIOLATED A CARDINAL RULE IN AVIATION -- 'FLY THE AIRPLANE.' I WAS DISTRACTED FROM MY DUTIES BY NO ONE'S FAULT BUT MY OWN; AND IT CAUSED US TO FLY OFF COURSE AND DANGEROUSLY CLOSE TO RSTR AIRSPACE. I HOPE IT WILL NEVER HAPPEN AGAIN.