Whenever the filmmaking process is divided into percentages (1 per cent inspiration, 99 per cent perspiration, that sort of thing), it's surprising that such factors as luck or necessity are excluded altogether from the equation. There's a fantasy, one to which I am highly susceptible, that everything which ends up on screen was always meant to be there. Blame it on auteur theory, or simply a child-like faith in the miracle of movies, but there it is.

Occasions when this has not been the case tend to be filed away as freak exceptions. -- think of the devastating final shots of Roman Polanski's The Ghost and Mike Leigh's Naked [2], both of which were invented on the hoof after shooting had begun. (In fact, Leigh always adheres to this process, as he explained to Amy Raphael in her excellent book Mike Leigh on Mike Leigh [3]: "[T]here's a big difference between the sort of ideas you have before you start shooting a film and what sort of creative process goes on when you've shot 99 per cent of the film, your cinematic juices are flowing and you've got the hang of the film you're making.")

But it was helpful this year on two separate occasions to have myself disabused of the whimsical notion that a film is ever born fully-formed. In these instances, my eyes were opened by two female directors, Kelly Reichardt and Andrea Arnold. (Quite coincidentally, both had just shot their newest movies in the old-fashioned and almost-defunct 4:3 aspect ratio, which produces a square rather than rectangular frame.) Back in March, I met Reichardt to discuss her stark western Meek's Cutoff [4]. One of the most contentious parts of this deliberately intense and slow-moving film was its elliptical ending, which leaves the audience to speculate on what might lie in store for the parched, desperate characters trudging along the Oregon Trail. The enigmatic final shots are so integral to the film's mysteries that I was surprised to learn that a different ending had been planned right up to the day of shooting. Reichardt told me:

The film actually ends a little differently than the script. The sun went down before we got our final shot on the last day, and I came back home without an ending to the movie, which is really devastating. I had to rearrange it in my mind. We didn't have the money to go back out there with all the actors and the animals and the wagons, so it had to become something other than what it was designed to be. I have this little prayer I say, where I tell myself that the lack of means is somehow working in my favour. Often it's true, and it can lead you some place good. In this case, it led me to an ending which was more suited to the film.

You have to be malleable. When I'm making a film, I want the script to be as far along as possible. But then all through scouting locations, the script is constantly changing to fit the places where things will happen. Then the actors come along. Things you've imagined being said in a certain way come out of their mouths sounding completely different, and I don't have a lot of rehearsal time so you have to adjust to that. If you knew how it was all going to be, then it wouldn't be such an interesting process. You read about Hitchcock, and how he said that shooting was boring because you knew everything that was going to happen, but then he wasn't working with our kind of budget or locations. I'm a big planner but it's still important to be open to whatever the day offers you when you're there.

I got an even greater surprise last month when I met Andrea Arnold (whose new version of Wuthering Heights I review in the next issue of the NS). I commended her on a particular moment in her 2006 debut, Red Road [5], which I consider to be one of the most imaginatively suggestive shots in recent cinema: a CCTV operator trains her camera on a suspicious-looking man who darts suddenly into the long grass on a stretch of waste ground, only for a lean fox to emerge a few seconds later from the other side of the undergrowth, creeping across the deserted road and vanishing into the night. The intimation, of course, is that a metamorphosis of some kind has occurred. I may actually have gasped when I saw that in a cinema for the first time, and I think I gasped again when Arnold revealed to me that the shot was entirely unplanned and fortuitous:

It was an accident; it's not like we had a fox there in the grass, ready to be released! When you're making a film, you often get less than you expect because what you start out with in your heart is such a pure thing, and the obstacles during production can be so immense and brutal. But, every now and then, you get so much more than you could ever have imagined. And the shot with the fox was one of those moments. To see it come out of the grass -- it was such a pure moment. I was delighted! I knew it was going in the film, no matter what. And I'd already written into the script the scene where the characters hear the fox noises later on. If you live in London, as I do, then you hear those awful noises all the time; it's just the strangest sound, like babies being murdered, and I wanted that in the film. The CCTV footage was shot before everything else, and it all just tied in beautifully. When that happens, you're, like, [she raises her eyes heavenwards] 'Thank you!' You feel like you've been given a big present.

And so do we, sitting gobsmacked in the stalls.

"Meek's Cutoff [6]" and "Red Road [7]" are available both on DVD. "Wuthering Heights" is released in cinemas on Friday.