General:
* Tremulous: This is a common choice for a general sense of quivering, often implying nervousness or fear.
* Shaky: This implies instability or weakness.
* Trembling: Similar to "tremulous" but emphasizes rapid, small movements.
* Shivering: This implies a rapid trembling, usually due to cold or fear.
* Vibrating: This emphasizes a rapid back-and-forth movement.
Specific to Emotion:
* Fearful: If the quivering is due to fear.
* Nervous: If the quivering is due to anxiety or anticipation.
* Excited: If the quivering is due to excitement or anticipation.
Specific to Physical Action:
* Tense: If the quivering is due to muscle tension.
* Unstable: If the quivering indicates lack of stability.
* Spasmodic: If the quivering is jerky or irregular.
The best adjective to use will depend on the specific context and the nuance you want to convey.