What do other 2 pilots do if a plane goes through an emergency situation?

If a plane with a 2+2 aircrew set (long haul flight) goes through hydraulic loses or loses engine power, what do other pilots do? Do they just sit in the cabin and hope the pilots in the cockpit save the plane or run into the cockpit and help them out?

Best Answer:

Skipper747: There are 3 pilots on flights over 8 hours up to 12 hours long –
There are 4 pilots on flights over 12 hours long –

In the old days, pilot-copilot-flight engineer crew of 3 were flying up to 12 hours legs –

A captain occupies a pilot seat for takeoff, initial climb, final approach and landing –
A captain is always present in the cockpit (pilot seat) during cruise –

There is always a PF "pilot flying" and a PNF ("pilot NOT flying") in each team of 2 pilots –
One flies the airplane (in emergencies) and the other perform emergency check lists –

In emergency, an experienced pilot might join the pilots in cockpit for assistance –

Resting is REQUIRED for "heavy crew – 3 pilots" or "double crew – 4 pilots" –
Two pilots must be rested for the more critical part of the flight (approach-landing) –
There are specific flight time and duty time or rest rules and limitations to observe –

There is always a captain-qualified pilot present in the cockpit during flight –

Other answer:

They will offer assistance, but most likely will not be able to do much. You see, there are two sets of controls, a 3rd person would not be able to add much in terms of being able to do something. There was one occurrence, United Airline 232 back in 1989, where an off duty pilot was able to provide assistance manning the thrust handles while the pilot and co-pilot were wrestling with the control columns, but those are very rare events indeed.
The pilots flying the aircraft are sufficient to handle the situation for most emergencies.
Robert S:
Typically one or two of the pilots are only cruise-relief pilots. They would vacate their seats in an emergency and the captain would assume control of the aircraft.
They call in control room to get information about the situation.
Skoda John:
The flight deck crew will request them if they want help. There is not a lot if room in the cockpit.
They would go forward to offer assistance and advice.