The thrust reverser half, if I remember, weights 750lbs, to open it a hydraulic pump is used to operate a hydraulic actuator to pump open the cowl, once it's at a certain height a stay is fitted to prevent it closing whilst working in there.
Some more modern a/c have an inbuilt hydraulic motor and once you unlatch the cowl you press a button and up it goes.
It would of been an engineer, they wear epaulettes, and a mechanic.
Modern a/c have a communication system that makes the engineers aware of any problems on an inbound flt so they can be prepared to do whatever to fix or lock out something on the a/c, saves time and potential delays.