On a twin with most large turbofan engines, if the reverser deploys in flight your chances of a happy outcome are pretty minimal. I was involved in the Lauda investigation - including wind tunnel testing with a reverser deployed. It's not just the thrust asymmetry - the big problem is that the reverser efflux effectively spoils the lift on the wing and that side drops like a rock.
As a result, the cert basis for all Boeing aircraft has changed from 'it's controllable' to 'it'll never happen (3 independent failures, probability less than 10E-10 per flight hour).
It's a regulation that, with the reverser in an uncommanded position, the engine can not provide more than idle thrust. On the pre-FADEC engines this was done mechanically with a fairly complex feedback mechanism that would move the throttle to idle (rather forcefully as sheppey notes - yes it'll break your hand if it's in the wrong place). With FADEC it's all done with electronics and the throttle doesn't move but the engine will still be commanded to idle. The main problem - as Lauda demonstrated - is the transient. When Lauda deployed, the engine was at max climb and the decel characteristics at 24k/Mach .78 meant the aircraft was long since out of control before the engine reached idle.
Paradoxbox, Boeing displays an amber/green "REV" above the EPR/N1 display to indicate the reverser is in-transit (amber) or deployed (green) - and it's a robust indication. IF the reverser has in fact deployed it's not likely to help much.