The hunky 36-year-old has been dating jeweller Morgana MCNelis since 2005, but he jeopardised their solid relationship when his promise of writing her a letter a day while he was shooting the action picture on location fell by the wayside.

"We went to Colombia, and the mail system doesn't really work very well," he explained to Entertainment Weekly. "It's completely unreliable. I received a letter from her, and I realised that from the tone of it and things she was saying that she hadn't received the two letters that I sent before."

He then concluded it was best to stop writing entirely, "which obviously makes me sound like a total b**tard, appropriately so," he admitted. "But I was very apologetic."

His distant connection with Morgana over the five-month shooting period actually proved to be beneficial for Charlie, explaining the silence allowed him to dive deeper into his character Colonel Percival Fawcett, a real-life British explorer who became an absentee father while searching for the mythical titular city in the Amazonian jungles during the 1920s.

"One of the things that (director James Gray) and I felt strongly that we wanted to explore in the film was the conflict between family life and the exterior demands that are made on us and the social and economic demands that we all need to deal with in life, being balanced or in conflict with the internal drive, to be the people we want to be and achieve the things that we want to achieve," he shared.

Although the method-acting technique Hunnam adopted on set upset his partner, he was able to get their romance back on track by wooing Morgana with a special gift.

"I had the benefit of shooting in Colombia, where they have rather lovely and somewhat cheap emeralds," he smiles. "My girlfriend is a jewellery designer, so I was able to come back with an appropriately sized gift. It didn't remedy all of the trouble I was in, but it got me halfway there."

The Lost City of Z, which also stars Tom Holland and Robert Pattinson, reaches theatres in April (17).