Is that a great idea, or just a pile of rubbish on the street?

The trick to creating a hit movie is knowing when to keep pushing, keep investing, keep trying to make the idea work out, or when to give up.

Late one night, I was out walking with my dog. In Los Angeles - as in other big cities - dog owners are required to clean up after their dogs. Even in Paris, once famous for its streets and sidewalks dotted with dog droppings, people are starting to get the message.

So I take my dog out as late as possible, when there are fewer judgemental neighbours lurking about, fewer eyes to notice that sometimes - just sometimes, OK? - I let her do her business in an out-of-the-way spot, take a furtive glance to my left and right, and if no one is looking, I hurry quickly along.

That night, though, people were looking, so I dutifully removed the plastic bag that all dog owners are legally required to carry and I scooped up the offending matter. But as I did, I noticed something else on the ground.

There were two index cards, of the usual size. And there was writing on them.

I know that some writers - not me - use index cards to keep track of their story moments in a screenplay as an aid to rethinking and reworking the structure of their script - I try never to rethink or rework, if I can help it. I was naturally curious to read what some other writer had on his or her index cards. So I scooped them up too to see what this other writer had been working on before his index cards went spilling onto the street.

I wasn't planning to steal the ideas, of course. I was just curious.

Here's what was on them.

On one card, there were just the words "Act One" in blue felt tip. I guess that was at the top of the stack. On the other card, there was this: "Prologue. Space Launch. Exterior New Mexico Desert. X Prize launch. Ship crashes. Harper watches father die. End teaser."

Of course, because I'm a writer and naturally hate every other writer, I read the card to myself in a snarky interior tone of voice. And then I rolled my eyes and thought: "Boy, that's a stupid-sounding script!"

But there's nothing really about that card that sounds bad. (There's nothing in it that sounds good, either, but that's not the point of the card. It's just a writer's way of keeping track of the scenes in a script.)

Lots of movies start with a kid watching a parent die. Some of them are bad and some of them are good, but only a sneering fool would instantly turn on the eye-rolling tone of voice. Even if he was just talking to himself.

I mean, look, you can describe a lot of great movies and hit TV shows that way. All you have to do is boil a plot down to its essence, then add a sarcastic rejoinder.

Vicious shark terrorises Cape Cod island? Oh, yeah. Real scary.

Silent movie star hits the skids? How original.

Exterior. Downton Abbey. The butler hits the dressing gong. End teaser. Wow. Really breaking new ground there, huh?

See how easy it is?

There I was, standing on the street in the middle of the night, feeling superior. But I was the one picking stuff off the ground and reading it. I was the one with a couple of dirty index cards in one hand, and a bag of dog waste in the other. Who, exactly, should have been embarrassed in that scenario?

The entire multi-hundred-billion-dollar entertainment industry totters on top of a lot of ideas and index cards and notions that have all, at one time or another, sounded stupid. In fact, the question we constantly ask ourselves in this business - no matter what we do is this: is this thing I'm watching or reading just something that sounds stupid, or is it something that really is stupid?

Most of us - even the very best of us - never really know until we've already spent the time and all of the money to find out for sure.

It can't be that different in any other business, either. Every great idea starts, at some point, with an index card and a few jotted-down words. And every great idea, at some point on its way from notes on a card to product on the shelves, goes through an awkward, clumsy adolescent phase.

The trick to every hit movie - and probably every giant fortune - is knowing when to keep pushing, keep investing, keep trying to make the idea work out, or when to give up and toss the cards on the ground for someone else to find while he's walking his dog.

The rocket crashes? Harper watches his father die? You know, that could work.

Rob Long is a writer and producer based in Hollywood