For the B737 Classics, the thrust lever for that engine will close to idle immediately without pilot action. This takes place so that only idle reverse is experienced and is easily countered. There will be some buffet. Follow the QRH once you have the aircraft under control. If roll is severe due to high power on the good engine, immediate closing of the thrust lever on the "good" engine should minimize the severity of the roll
I understand in the B737 NG series the engine also automatically goes to idle reverse but the thrust lever doesn't move until you go through the QRH and take the published action.
I recommend you Google the report (see link) on the Lauda Air Boeing 767 that experienced an uncommanded in flight thrust reverser during climb. On that accident the aircraft rolled rapidly and control was lost. I recall reading that investigation showed the crew had four seconds to take corrective action before loss of control would occur. That corrective action was not specified but I presume would have included immediate closing of both thrust levers and knowledge of unusual attitude recovery on instruments.
See:
https://en.wikipedia.org/wiki/Lauda_Air_Flight_004
How could you practice for a catastrophic event in a simulator?
Depending on the type of simulator, the uncommanded in-flight thrust reversal should be selectable on the simulator instructor station