I worked as a cruise pilot in the past, here's a typical flight:
Turn up in briefing room, prepare all documents with F/O, do the chart (we did Oceanic via NATS so needed a chart with ETOPS circles). With capt's permission notify final fuel. In my experience 40% of capt's consulted and involved you, 40% didn't and the final 20% didn't have a clue how to act in a heavy crew.
On board, program FMC from PF chair as they went to do the walkaround. Get clearance and fill out V speed card. W&B document was my responsibility to complete in outlying airports.
After take off, fill in waypoint times, notify hosties of expected turbulence so they could plan the service. Plan the work schedule based on who was doing landing and who was tired at present.
Rarely got into a seat within 30 minutes of take off, some captains saw it as integral you were involved as much as possible. Some went down the back for a sleep, some never left their seat.
Generally do most comms, most inflight paperwork and weather dictation. NATS you have 5 minutes of activity followed by 1 hour of reading the manuals or paper. Other than that, a 12 hour flight was 2 hours sleeping, 1 hour eating, 1 hour toilet/stretch legs in aft galley/talking to crew, 3/9 hours in a seat.
The company I worked for had an SOP that whenever a CP was in a control seat, they would defer to PF regardless of their role before.
20 minutes before TOD, back into back, look to speak to Ops and give them ETA and get stand, finish up paperwork that you could without getting final Landing/Block times.
After landing, pass all paperwork to capt for signing, prepare new documents for incoming crew and tidy up the mess.
Hotel after meet and greet with incoming crew.
GOOD:
Easy life, good to absorb procedures from the third seat "the throne". Get a decent layover 4x a month (mini holiday - if its a crap crew it drags). 11 colleagues to go away with, 11 new colleagues a week later.
BAD:
Job in mundane, treated as a dick/skivvy by a lot of colleagues (including hosties), could be messed about with by management with respect to upgrade as its an inconvenience for them. Some regulatory authorities and airlines don't value all the time logged towards their thresholds. No T/O or Landings.
Only a fool enters into a job without researching it. If it works for you financially or as career progression then you apply, if you have an aversion or don't fancy it, you don't.