tremble at (something)
To feel profound excitement, anticipation, anxiety, or fear from the thought of some action or thing. You'll never get anywhere in business if you tremble at the idea of doing something risky. We all trembled at the thought of having that much money at our disposal.
tremble at something
to shake with fear or anticipation at the thought of something. David trembled at the thought of having to go to Russia by himself. Carl trembled at the idea of winning first place.