The biggest mistake screenwriters make is - they come up with an idea on a Monday and decide that's going to be the script they're going to spend the next three to six months working on, rather than spending an equal amount of time going through lots of ideas and making sure the one they're going to write is tested, critically received by lots of people and then, when they know they really have something strong, they sit down and spend the time writing it. They work and sweat and bleed on screenplays that are wrong-headed to begin with. It may have good writing, but the idea, story, and concept aren't that commercial or strong and thus, will never sell.
Of course there are those who would say that you have to “follow your passion” whether that leads to a commercially viable screenplay or not. I certainly thought that way for a long time, and I’ve ended up with far too many projects either unfinished or unproduced. And not because I can’t write or because I’m unfamiliar with screenwriting conventions.
The point of the matter is, that writing a good screenplay takes a huge amount of effort and perseverance, and it really only makes sense to put in that work if the idea at the heart of the screenplay is genuinely well thought through. That is, at least, if you’re serious about earning a living writing screenplays.
Does that mean that the only screenplays worth writing are clones or imitations of successful Hollywood movies? I don’t think so. I think the main criterion should be: Is there potentially (and realistically) a market for your movie idea? Does the screenplay have at its core a unique enough idea, or an intriguing enough twist on a familiar genre, to pique an audience’s interest?
I hate dealing with this issue, because it brings up the whole question of whether film is primarily an art form or a business enterprise. And like most screenwriters, I like to think that what I write has some relevance, that it’s more than “mere entertainment.” The misconception being that an entertaining movie is by definition superficial and vacuous.
Increasingly, I’m convinced that time spent testing and selecting ideas for screenplays before committing to writing a screenplay, is time very well spent. And although writing anything is good practice, it’s a pity to spend months or years writing a screenplay that has no real potential of being produced.