The actor was drifting through life in his late teens before he became a volunteer at the Winter Olympics in New York in 1980, where a woman asked him to look after her ageing parents in Sweden for three weeks.
When he arrived he found the pair was confused and embroiled in a series of strange habits - and Mortenson found himself playing a character to help deal with them.
He tells Britain's Seven magazine, "When I was a volunteer at the Winter Olympics, I met a woman who asked me if I would go and look after her elderly parents in Sweden for three weeks. They were in their nineties, and they lived miles from anywhere.
"It was one of the strangest experiences of my life. Every morning, the old man would write a letter to the king explaining why he couldn't pay his taxes. The wife was obsessed with Bjorn Borg and with playing bridge. She would insist that I play with her, even though I had no idea of the rules. Also, you need four people to play bridge, of course.
"Every night we would sit down and pretend to play. We had imaginary partners and she would deal out the hands... It was totally crazy, but it was one of the first times I had to try and pretend to be someone else and in an odd sort of way, I found it fulfilling."