It’s an old saw in Hollywood that you shouldn’t put your own money into a movie. But I’ve always had a powerful respect for anyone who does. Clearly, it’s a sign of their commitment — that they care enough about what they’re doing to have some skin in the game. I also think there’s an intoxicating roll-of-the-dice payoff in play: If you put your own money into a movie, your investment could hit the jackpot. (That’s what happened with Mel Gibson and “The Passion of the Christ” and George Lucas with “Star Wars.”) And, of course, there’s the admirable idea that those who self-finance are trying to bring a film into the marketplace that a corporate studio said no to. That’s one way that motion pictures can stay adventurous.

So the reckless and committed bravado that Kevin Costner demonstrated by pouring $38 million of his own money into “Horizon: An American Saga” is something I can get behind. Actually, as Costner finally confessed, it might be closer to $50 million; maybe he was initially feeling a bit shy about owning up to that quixotic level of personal investment, since (to repeat) you’re not supposed to do it.

But obviously, Costner can afford it. All the players who do this sort of thing can. They have plenty of assets left over. (That’s one reason I’m surprised it doesn’t happen more often.) Costner has always been the definition of a star who cares, who acts in and directs worthy projects, who possesses a reverence for the art of movies. That he poured his own money into a sprawling Western magnum opus has a kind of purity to it.

That said, it would be hard to think of another example of a movie that proved the old saw right as much as “Horizon” does.

The box office returns are now in on “Horizon: An American Saga — Chapter 1,” and taken in a big-picture way, as an indicator of where the saga is heading, the numbers are not very pretty. I suspected they wouldn’t be as soon as I saw the film at Cannes, where I was one of many critics who gave it a mixed to withering review. What made this a special case is that the three-hour film is just one-quarter of the total project. “Chapter 2” is set to come out in August. “Chapter 3” is now being shot. And “Chapter 4,” at this juncture, is a frontier castle in the air — an idea for a movie hanging out on Kevin Costner’s balance sheet.

What this means is that “Horizon,” with a likely $12 million opening weekend and a $100 million price tag (not for the whole saga; just for “Chapter 1”), has the potential to be not only a financial train wreck, but the kind of train wreck that unfolds in slow motion over the course of months. In the case of “Chapter 3,” it involves building the train even as it’s in the middle of crashing. That’s a lot of box office pain to buy for $38 million.

Costner knew what he was doing when he took his gamble, and he’ll be fine. Maybe the money will even come back to him through residuals. But when a big movie opens with this kind of thud, it’s worth asking what happened, and if there are lessons to be learned. In this case, there’s a major lesson. It is this: Don’t turn movies into television.

Because that’s what Costner tried to do, and that was his folly. His star had dimmed in the 2010s, but it came roaring back with “Yellowstone,” the Taylor Sheridan TV series that began in 2018. It’s obvious that Costner took his cue from the show’s extraordinary success in deciding to craft “Horizon” as an episodic drama, one that will (theoretically) stretch out to more than 10 hours. It’s not a 10-hour movie, exactly. It’s a series — or, as I described “Chapter 1” in my review, “the seedbed for a miniseries.” Because there’s a feeling you have watching “Chapter 1” that you’re being introduced to all these characters, but you haven’t even gotten to the good stuff yet. That sort of thing can work on television. But in the movies it’s fatal. I would argue that a movie has to hook you in the first 40 minutes or it’s toast.

Is the problem with “Chapter 1” that it isn’t a better miniseries? Possibly. But I still think the issue is one of form. The discursive, anecdotal, all-over-the-place drama of “Horizon” feels, at least in the first three hours, like homework, and I don’t think that’s because I’m not enough of a “red-state audience person” to get it. It’s because television writing is different from movie writing. TV episodes, especially in an ensemble piece, are often open-ended. There’s a dip-in/dip-out quality to them. Whereas movies require a sense of resolution. And there’s a hubris built into the four-chapter concept of “Horizon,” since if people don’t turn out for “Chapter 1,” who in God’s name is going to care about “Chapter 2”? That audience, in August, is likely to be even smaller.

Of course, there’s always the possibility that you won’t need to have seen “Chapter 1” to enjoy “Chapter 2,” or that “Chapter 2” will prove to be better. Maybe that movie, two months from now, will take on a life of its own. But I don’t think that’s how Kevin Costner envisioned it. He wants his audience to be all in. And he has always been a leisurely dramatist. (The original cut of “Dances with Wolves” was five hours long.) Investing your own money in a movie is, to my mind, an honorable and even courageous thing to do, but the problem with the level of investment Costner has poured into “Horizon” is that he has already made the movie more important than any movie should claim to be. The drama of whether audiences show up for it now transcends the drama onscreen. Sure, “Horizon” is “big” and “sprawling” and “epic,” but thus far it’s busy and fragmented rather than grand. I suspect that, one way or another, it will ultimately end its journey on the small screen: the place it was really always meant to be.