Johnny on the spot

I don't remember when I first started using the phrase "Johnny on the spot", but it's one of my favourite sayings. I use it sparingly, and try to use it appropriately.

The grammatical genesis of ‘Johnny on the spot’ cannot be traced very clearly, but the phrase certainly originated from the longer and less expressive one, ‘Johnny is always on the spot when wanted.’ ... The expression is to some extent a variation or rather a continuation of that other phrase, ‘He gets there.’” Johnny here must be a general name for any young male and doesn’t refer to a real person.