Johnny-on-the-spot
Johnny-on-the-spot {adj. phr.}
At the right place when needed; present and ready to help; very prompt; on time.
A good waterboy is always Johnny-on-the-spot.
The firemen were Johnny-on-the-spot and put out the fire in the house soon after it started.
Compare: ON THE JOB.
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Source: A Dictionary of American Idioms