Blackmailed
As soon as he calmed his nerves and thought about the situation properly, there were more clues that he could piece together to make the situation make sense. Iris told him—that the twin rulers' whiskers would show the emotions they were currently experiencing.
It was the side-effect of the incense burning inside that room.
From the start, Rex observed and made mental notes about their whiskers.
Since they could suppress their fears, being someone who came from a royal standing, using the bleak aura their bodies emitted as a measurement would be way too inconsistent. Micro-expressions were also no use.
Royals tend to be hard to read.
Mainly due to the fact that they were taught to show nothing about what they felt inside.
However, their whiskers don't lie—that's what Iris told him.
Rex could use their whiskers to determine their emotions.