Documentary series Long Way Up sees Ewan McGregor and Charley Boorman take on some of the world's toughest terrain, Danielle de Wolfe finds out more.

"It's a long time coming," notes Ewan McGregor of his reunion with fellow actor and motorbike enthusiast Charley Boorman. Twelve years to be precise.

Following on from the success of 2004's Long Way Round and 2007's Long Way Down, the pair's new Apple TV+ series, Long Way Up, documents their adventure across some of the remotest regions of South America.

"We'd always thought about doing this third trip," says McGregor. "When you look at a world map, we'd gone directly across it and we went down through the African continent, so there was this glaring other route through the Americas that was left to do.

"Charlie and I had drifted apart somewhat over the years since we did Long Way Down, due to the fact I'd moved to America and Charlie was very busy – he'd shot some other television shows and he was doing a lot of touring with other people on motorbikes.

"So, when I was working in London or I came over to visit family, Charlie was often just not there. There was no fallout or anything, we just sort of drifted apart."

It's often said that a near-death experience can act as a catalyst for change. In the case of McGregor and Boorman, it was a freak accident during a European trip that acted as the trigger for a long-overdue reunion.

"I think you'll see in the show, Charlie had a very terrible motorcycle accident in Portugal and I think it sort of reminded me that you can't let these important friendships in your life drift and you've got to look after them," says McGregor, 49.

"As soon as I was able to, I got to see Charlie in London and then we picked right up where we'd left off.

"It was like no time had passed. Poor old Charlie was wheeling around; it took a long time – he had an almost two-year recovery from that accident, which was really brutal."

The accident in question took place while on a press trip celebrating the launch of the new Triumph motorbike. The near-fatal encounter saw Boorman career into a wall in an attempt to avoid an oncoming motorist.

"It was touch and go as to whether I'd keep one leg," recalls Boorman, 54. "Thanks to the doctors, I did and I'm able to ride motorbikes again. But for me, it was always about my therapy – and my physiotherapy was all focused on the fact I needed to get back on that motorbike and have that freedom.

"I think that happens to a lot of people who ride motorbikes."

Armed with a rekindled desire to get back in the saddle and set forth on a new adventure, plans for a third instalment of the Long Way series quickly fell into place.

"It wasn't long before we were sitting in Charlie's house with Russ Malkin, our British producer, and David Alexanian, our American producer, and we started dreaming up this Long Way Up trip," McGregor recalls.

Conversation then turned to the potential for swapping traditional petrol-fuelled motorbike engines for a greener alternative. The idea? To put renewable energy to the test in some of the most remote regions on earth.

"When we decided to go electric, I don't think we wanted the electric motorcycle to get in the way of the adventure; we wanted it to be part of it. And I think we were both very excited about that," says Boorman. "They were all prototype vehicles that weren't really in production or weren't up for sale, y'know? We didn't get much of a chance to be able to test them," he added.

"When we finally got to South America to start our journey, Long Way Up, I think Ewan and I had only ridden the Harley Davidsons for a couple of hours and we'd never really charged them. So, it was a really steep learning curve for all of us."

There began a series of challenges brought on by the continent's dramatic terrain and diverse weather conditions.

"We started in the winter, which was not such good planning on our behalf," recalls Boorman.

"We realised that the batteries don't like it when it's very cold, so they don't perform as well. We had to calculate how fast we would go, as to how much time it would take, and then we had to figure out how far we could go each day. Once we finally decided that we'd be able to get to that point, we would try to find someone that would let us plug into their house."

The adverse weather conditions, coupled with the newly rekindled friendship, saw the trip transform into an extreme bonding experience.

In particular, a wide stretch of Bolivian desert, which proved "some of the hardest riding I think that we did on the trip. It really pushed us to the limit", recalls Boorman.

"I'd fall off and suddenly Ewan would be there helping me up and vice versa.

"It's very nice to know that your best friend has got your back and is there to pick you up - it's a very special relationship."

The first three episodes of Long Way Up will premiere globally on Apple TV+ on Friday September 18, and new episodes will then roll out weekly.