Luke var definitivt inte faking.
Per filmskript
Luke is almost unconscious beneath the continuing assault of the Emperor's lightning. He clutches a canister to keep from falling into the bottomless shaft as the bolts tear through him.
EMPEROR: Your feeble skills are no match for the power of the dark side. You have paid the price for your lack of vision.
Luke writhes on the floor in unbearable pain, reaching weakly up toward where Vader stands watching.
Filmen officiella novellering
On the Death Star, Luke was nearly unconscious beneath the continuing assault of the Emperor’s lightning. Tormented beyond reason, betaken of a weakness that drained his very essence, he hoped for nothing more than to submit to the nothingness toward which he was drifting.
...
Luke’s body slowed, wilted, finally crumpled under the hideous barrage. He stopped moving altogether. At last, he appeared totally lifeless. The Emperor hissed maliciously.
Filmens nya (och helt kanon) junior novellisation
The Emperor is right. Luke had no idea that the dark side was this powerful, that the Force could be used this way. He does understand now and it is a terrible thing to understand.
Luke collapses under the onslaught, clutching at a railing to keep from being pushed into the reactor shaft behind him. But he can do nothing more. The storm of dark energy blots out his own thoughts and makes his body jerk and writhe.
och filmen original junior romanisering
More blue lightning coursed over and through Luke. He fought to remain conscious and clutched at a canister to keep from falling into the adjacent shaft.
...
Luke had not imagined pain beyond what he had already suffered, but then he was hit by a wave of power that was even more staggering. His harsh screams echoed across the throne room.