Similar policy in Argentina as described by Intruder's above.
Extra crewmember performs walkaround. Participate in paperwork.
Taxi/takeoff and landing, extra pair of eyes for traffic. Monitor R/T.
Remain in cockpit until above 10,000 ft. Take turns in cruise.
xxx
And correct "operating pilots" if/when making mistakes.
Happy contrails
P.S.
For those many of you never making mistakes, tell extra pilot to shut-up.