B767-300 and 300 ER
Fatigue; Situational Awareness
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Flight Crew Returned To Clearance; Flight Crew Became Reoriented
Human Factors
[ATC] had given us a clearance to fly 6 miles right of track.  As Pilot Monitoring (PM); I read back the clearance and entered the offset into the FMC.  The Captain; Pilot Flying (PF) selected HDG SEL and turned toward the offset route.  He had been done this on each of his legs before (and I sometimes do the same) so I saw nothing amiss.  The Captain had his hands on the yoke so I went back to my iPad that I had open to study since I had recurrent training soon. I was not monitoring the intercept.  I did not notice that the Captain never selected LNAV.  In the dark cockpit I did not notice the Captains head droop slightly.  When [ATC] queried us as to our course/offset; I noticed the Captain's head come up and saw that we were well past our six mile offset.  I pulled up the PROGRESS page and saw that we were nine miles farther right than cleared. The Captain had already begun a change of course back to our cleared offset. The Controllers only comment was that we were headed out of his sector.  I lost sight of my duties as PM when the Captain kept his hands on the yoke for the intercept.  While on autopilot; I had not witnessed him keeping his hands on the yoke before.  I also did not notice that he nodded off during the minute it took to offset those six miles.  It took a matter of reading one page to miss the actions that should have been occurring. In light of this error; I will have to be cognizant of where my attention is drawn.  There is no reason to pay attention to other; non-flight things when a change is occurring over such a short time span.  Also; PM duties include everything associated with the flight; the other pilot included.
B767-300 First Officer; the pilot monitoring; reported a track deviation when the pilot flying failed to select LNAV during an offset maneuver.  Fatigue was cited as a contributing factor.