What's the exact moment were the spoilers are supposed to be deployed on a A320? Are they deployed when the levers are pulled back to 'IDLE' or only when pulled back to 'Reverse'? 

I still can't figure out, 12 years after, why flight TAM JJ3054 crashed into a gas station and a building, in São Paulo, Brazil. The investigation concluded that the captain pulled the left engine lever to reverse, but "by mistake", left the right one on 'CL'. 

For those who don't remember the tragedy: 1- right reverse was pinned due to a malfunction; 2 - It was raining at the Congonhas Intl. Airport at the moment of the crash; 3 - the a320 landed on the touchdown mark on 35L runway ( about 1290 meters long); 4- Left reverse deployed but no spoilers nor hydraulic breaks engaged; 4 - Death toll: 199 souls.
To this day, they can't fully determine whether the captain left the right lever on 'CL' or the FDR displayed, erroneously, 80% of thrust on right engine, when it was supposedly on 'Reverse' position. The pedestal was found among the wreckage, twisted and melted beyond recognition.
PL at idle is one of several conditions (weight on wheels, N1, wheel speed etc) that have to be satisfied to get ground lift dumpers to come out on airliners.  Reverse isn't required.  Once on the ground, just moving the PL levers back toward TO thrust makes them come back down.