Funny it may seem, but Saeril wasn't expecting this sudden outburst to come from the leader of the company. When Thorin began to speak in Khuzdul, she understood his every word, and this wasn't pleasing for the king nor to her. The illusion wasn't what frightened Saeril, in fact, the illusion only struck her with pain for the ElvenKing. 

"Wait a minute..wait! I only wish to speak with you, my lord!", she protested quickly as soon as her arms were taken by the guards. "About this treasure you so-called speak of", she said.