Antonyms for the phrase "was quiescent" include active, dynamic, lively, energetic, and bustling. When something is quiescent, it is quiet, still, and dormant. Its opposite, active, suggests that something is alive, energetic, and thriving. Dynamic implies that something is constantly changing, moving, and evolving. Lively means vibrant, animated, and full of activity. Energetic suggests that something is full of life force and vitality. Finally, bustling implies that there is a lot of activity and action taking place. These antonyms for "was quiescent" are useful for providing a clear and vivid description of a situation or object.