The Prophet - now that's a rockumentary

Filming on a cliff that towers up to a vertical kilometre in height obviously comes with a set of terrifying logistical problems. I'm a rock-climbing film-maker and my latest film, The Prophet, follows Britain's best big-wall climber, Leo Houlding, as he makes the first ascent of one of the hardest climbs on the world's most celebrated cliff: El Capitan in Yosemite, California.

I think the key to being able to film a climb like this successfully is a good working relationship with the climber. Leo is not only experienced in working in film and TV, but he also knows the value in getting an ascent filmed properly. The thing about filming climbing is that the money-shots are always from above the line the climber is on. Slightly off to one side, ideally, to create a bit of an angle, and you want to be out from the cliff to get a sense of perspective. Getting in the right positions can be tricky. This may require the climber to climb some sections twice, as was the case when I filmed Leo on the face of Mount Asgard in the Arctic wilderness of Baffin Island.

Fortunately, whilst El Cap is just as imposing as Mt Asgard, it is a lot more accessible and nowhere near as cold! This makes big-wall filming much less problematic.

With no time to adjust, I find the experience of going over the edge absolutely nerve-shredding every time. Suddenly your whole life is dependent on one rope just 10mm in diameter. There's no back-up: if something goes wrong, you're in for the worst ten seconds of your life before it ends...

You need to have plenty of supplies and a good place to sleep at the top of the cliff (the climbers sleep on the cliff, but I go up the fixed ropes at the end of each day). The other golden rule is not to drop anything, of course – particularly yourself.

Kendal Mountain Festival, to 21 November (www.mountainfest.co.uk)