Thor: Dark World not exactly spoilers, amusing speculation
"If you look in the cell and he seems to be lying there dead, or is having some kind of mysterious illness, or being attacked by anything, DO NOT OPEN THE DOOR TO HIS CELL. It's *almost certainly* an illusion, and as soon as the door is open he'll escape."
Thor: Dark World not exactly spoilers, amusing speculation
"If you look in the cell and he's dead, wait for him to start decaying. Call in an expert who can tell you if the decomposition of his corpse is accurate. If it seems to be legit, DO NOT OPEN THE DOOR TO HIS CELL. Mark him dead and don't bring any more meals. He'll either drop the illusion out of hunger, or continue to be dead, and either way it's probably safest to just leave him there."