"Johnny-on-the-spots" is commonly used to describe someone who is quick to respond or jump into action. However, there are several other words that can be used as synonyms to convey a similar meaning. For instance, the word "nimble" can be used to describe someone who is quick and agile. "Prompt" is another synonym that is used to describe quick and immediate action. "Alert" also conveys a sense of quickness and readiness. Other synonyms include "swift," "expeditious," and "speedy." In essence, these words can help to describe someone who is always ready to take action and respond with urgency when needed.