Airline, but the type is used as business jet as well (B737). The right hand seat pilot selects the engine start switch to GRD which engages the starter, the left hand seat pilot will add the fuel (engine start lever to idle). Both pilots have to be in their respective seats for engine starts, usually it is done during pushback, only on taxy-out positions it is done on position.
Before engine starts all passengers have to be seated, external and internal doors have to be closed and all checklists up to that point as well as the departure briefing have to be completed.
But i guess airline and GA differ in that phase quite a bit depending on the guest in question.