The Good Will Hunting actor was walking through a London park with his wife Luciana and one of their daughters when he spotted a couple approaching.

Assuming he was about to be asked for a photo, Damon graciously put his arm around the woman before realising to his horror that they did not know he was famous and merely wanted him to take the picture.

Damon tells U.K. talk show host Graham Norton, "I was in a London park with just my wife and daughter. We were alone in the park when a young Spanish couple came along, saw me, had an exchange and then pulled out a camera. I said to my wife, 'Here we go.'

"It was only after I put my arm around the woman to pose for a picture and the husband gave me an odd look that I realised they had no idea who I was and just wanted a photographer to take their picture! I was so glad my wife was there to witness that."