B767-300 and 300 ER
Flight Crew Instrument; Flight Crew Commercial; Flight Crew Multiengine; Flight Crew Private
Distraction; Physiological - Other; Other / Unknown; Situational Awareness
Ground Event / Encounter Loss Of Aircraft Control; Ground Excursion Taxiway
Human Factors
Uneventful flight to ZZZZ ILS [Runway] XXL approach and landing. Upon landing we taxied clear of Runway XXL at L2 with no issues. Followed taxi instructions provided by ATC to continue taxi L2; L and cross XY/XZ. After turning right onto Lima at slow speed and on taxi centerline aircraft was pointed directly into the sun and the Captain reached down and grabbed a checklist from the aircraft dashboard. As aircraft continued on taxiway Lima and Captain placed checklist in window to block the sun he reached back down to grab the tiller and the aircraft veered to the right and then immediately seemed to accelerate to the right off the centerline. As this rapid movement happened the Captain yelled 'make it stop' and as the Pilot Monitoring I was already racing to put my feet onto the brakes well before departing the taxiway. The momentum of the aircraft was too great and the nose gear settled off the taxiway. During this time we were both standing on the brakes and the Captain had minimum reversers selected as a last ditch effort to stop the aircraft prior to departing the paved surface. We secured the right engine after departing; started the APU and secured the left engine. Towed back to ramp successfully. Improper use of tiller. While taxing keep both hands at all times on tiller and throttles.
B767 First Officer reported the Captain used improper taxi procedures causing the taxiway excursion.