I was pleased to read Karen's comment, because now I can finally admit that 'Johnny on the spot' is new for me, too.

This post is the first time I'd come ever across this expression.

I've googled it (and read the Wikipedia page on a little-known 1950s British film of that name) so am prepared to admit that it exists, but I had honestly never heard it before. The fact that Tim, Dan and Chris are familiar with it makes me think it might be more of an American expression.

That said, it makes a lot of sense. Even as a complete newcomer to this idiom, I can see the difference between 'Johnny on the spot' and 'Jack of all trades': the former is prompt and available, while the second is versatile (but not necessarily competent in any particular skill).

Su.ki. I read on Dict.cc., while searching for some sort of German translation of „Johnny on the spot“ (having never found a translation I liked that I thought conveyed the meaning adequately) that the phrase was, as you stated „US, informal, dated.“ That it was US and informal didn‘t surprise me, but to call it dated I can’t agree with. I think that’s taking it a bit too far. I’ve heard „Johnny on the spot“ my whole like, not terribly frequently, but not so infrequently to call it dated. It’s doesn’t sound like 1920s/1930s English to me (while there are a few phrases my 92-year old mom uses I would definitely say do sound dated).

Miriam brings up a good point I’ve noticed myself, when she stated, „the funny thing is that no matter what phrase I post, there are always native speakers who say that they never heard the phrase, others who say that it's dated and others say that it's very common. British English speakers often never heard of American English expressions and vice versa. Why is that? Do British people not watch American films and vice versa?“ Great question.

I‘m in a bit of a debate about the use of „Californian“ to use in reference to things (as opposed to people) right now with another Californian. Like I said, it‘s fine to say with respect to people, but I’ve never heard „Californian“, with the redundant „n“ at the end, used with regard to a thing, such as „a Californian city“, except by people from Britain, Australia or New Zealand, and seeing the same appear in British newspapers. Yet, I have a guy saying he lives in Central California and had heard „Californian“ refer to things, like a city, by native Californians before. I’m like, „You’ve got to be kidding me. Who talks like that here?“ Now that really confused me because that sounds very British to me. Maybe it’s because I’m in the San Francisco Bay Area in the North, and he’s on the Central Coast of the State, but there can’t be that much of a difference. I can’t figure it out.

The Free Dictionary defines it as US, informal, dated, which seems to summarise what we've already said.

But I did find a couple of portable toilet hire companies with this rather apt name!

Both expressions are quite familiar in the US and I’ve heard them quite frequently. Especially “Jack of all trades.” “Johnny on the spot” I’d say gets far less use, but you do hear it enough, and it is definitely not synonymous with “Jack of all trades.”

Dan and Chris gave good answers, by the way, but I’ll give you my take on it (even though I think the TV scriptwriters didn’t choose the best expression for the context in saying “Johnny on the spot.”) Particularly when it’s followed by “had that one locked and chambered”, an expression I’ve never heard before.

Let’s say I’m having a handyman, John, who is a Jack of all trades (can fix or repair anything) come to my house at 8:00 to paint the wall in my garage. And, assume I’m really expecting that he’ll show up at roughly 8:14 or so. But, say instead he shows up at 7:59. My neighbor asks, “Did John show up and paint your wall?” I might respond, “Yeah, he told me he’d be there at 8:00 and he was Johnny on the spot. Got there at 7:59.” That’s how I’ve heard the phrase used or would use it.

Like Dan said “Jack of all trades” by itself just means a versatile handyman (not a professional person). It’s neutral in reference to his skills. But, I’ve also heard “A Jack of all trades and master of none” which means he’s not much good at anything he does, or is at best mediocre. So, that way of saying it does carry a rather negative association.

Johnny on the spot conveys to me just a promptness, or readiness, a timeliness. Reverso.Context is pretty good for German translations, and these two really seem the best to me:

As Tim said, the terms have different meanings.

One meaning of Johnny-on-the-spot is "being present [on-the-spot] unnaturally soon." To me, the TV show uses the phrase incorrectly. The bank official has an instant answer to an unexpected question which implies that he is involved in the robbery and has mentally rehearsed answers to questions that the police might ask.

A traditional example of Johnny-on-the-spot would be a news photographer or paparazzi who shows up at an accident before the police.

Police: Joe Smith! You're a regular Johnny-on-the-spot! How did you get here before us?

Joe Smith: Well ... I was close by and I was monitoring the police radio, so I got here in two minutes.