Series producer/director John Paul Davidson on crisscrossing the United States in a London taxi.

Commissioned by Richard Klein and the then controller of BBC1, Peter Fincham, the decision to make a travel series with Stephen Fry taking a London cab through all 50 states of the USA was reached just over a year ago. It was always going to be a tight schedule - given the distances involved and Stephen's other commitments.

Agreeing the route was the first decision to be made and would define the six months of filming on the road. With the entire canvas of 50 states to fill, deciding where to film and as importantly who to include in the films were the major research tasks. We also needed to work out how to reach each and every one of those states in some sort of continuous journey.

People tend to think of the USA as primarily the two coasts and in fact the east-west axis is a more travelled route than the north-south one. But given that we would be filming a large number of the states in winter, the cunning choice was to make the journey from north to south, where the temperature differences would be more extreme.

In episode three, for instance, we started filming with Mardi Gras in New Orleans at the beginning of February, when the temperature was a balmy 25oC and ended three weeks later with ice fishing in Minnesota at -20oC. Around the halfway mark in St Louis we hit winter and from then on the snow was a constant feature. It drove home just how vast the country is.

The result is that we zigzagged up and down the country four times starting in Maine in the north-east and ending in Seattle in the north-west before hopping over to Alaska and Hawaii.

The other conceit of the series, apart from visiting every state, was that we were to travel in a London taxi. Stephen has driven one for many years and is a great fan of their dubious comfort.

They are terrific filming vehicles: distinctive, with plenty of headroom (designed so a gentleman wearing a top hat can fit) and large enough to carry three extra bodies so we could conduct interviews in it. It soon became clear that transporting one from the UK was fairly impractical: insurance, homologation issues and the stringent US Environmental Protection Agency issues over vehicle emissions made it way too complicated. Our excellent production manager, Janina, however tracked one down in Chicago. So it joined us in Eastport Maine for the beginning of the trip that started in October and ended in May, and covered 18,000 miles without mishap.

There are many advantages to making a road movie in the USA, and even more with a presenter like Stephen Fry. Driving is a must and luckily he enjoys driving as much as eating, reading, talking, drinking and fixing all sorts of software problems on a myriad collection of Mac products. So there we had one driver. We needed another vehicle to carry the gear and a crew of six: cameras, sound APs and stills photographer. Weirdly in a country designed for big cars and big roads it was tricky finding the ideal vehicle. After a couple of false starts we found a van designed for travelling rock bands, along with the perfect driver, New Yorker Frank Davis, who was ever calm, and refused to break the speed limit.

Actually I never travelled in it preferring the noisy cab and the ability to stretch out in the back being driven by the most erudite cabbie one could ever hope for, who didn't object if you smoked a cigar in the back, and best of all, didn't turn on the meter.

It would make better copy if we had had mishaps, misadventures, near death experiences, but in truth the shoots went remarkably smoothly in no small measure due to the excellence of the APs and our production manager back at base, and the calm competence of the crews and my fellow director, Michael Waldman.

Our only hitch came after finishing the first two shoots. Stephen went to Brazil to shoot another documentary and promptly broke his arm. Ten pins had to be inserted, which meant for the next stretch up the Mississippi he had his arm in a sling and found it very painful to drive long distances. Luckily it mended reasonably quickly but it did mean he couldn't drive a Model T Ford in Detroit (the gears were too stiff), or paddle a canoe along the Mississippi. But it also gave him the perfect excuse not to go surfing in Hawaii.
Stephen Fry in America is a DCD Media-owned West Park Pictures production for BBC1. It airs from Sunday 12 October at 9pm

John Paul Davidson: My tricks of the trade
As many cans of Red Bull and Starbucks Double Shot Espresso as you can bear for those endless interstate journeys. Beef jerky and trail mix for times when meals are far apart. And expandable trousers to accommodate all the meals we did have.

Take an iPhone for instant research and fact checks on the move. It saves a bundle if you get an American contract as the data downloads can rack up the bill.

A couple of blondes to charm everyone, including traffic cops, naval officers and Mormon missionaries.

GPS - brilliant at finding hotels in strange cities in the middle of the night, much better than any map.

A good supply of detailed maps - they provide a mass of information on one sheet and they don't lose their signals.