"Vaughn read somewhere that some of the more militaristic murals were being taken down in places such as Harryville in Co Antrim, and he came up with the idea of doing the documentary before they disappear off the landscape, " said a Sinn Fein spokesman.

"He has a production team working on the documentary over here, so he came over to help them out for a few days."

Vaughn met Adams on the Andersonstown Road in West Belfast at a mural commemorating the life of 1981 hunger striker Kieran Doherty. "The American crew were very interested in the murals and they are keen to do the documentary about the diversity of culture and politics that they represent, " the Sinn Fein spokesman said. "Gerry [Adams] spoke to them about the origin of murals and their importance in the context of political censorship in the past, when they were one of the few ways for people to portray their political expressions."

In east Belfast, Vaughn met David Ervine at a mural depicting workers arriving at the world-renowned Harland and Wolff shipyard.

"We did a spot of filming in Dee Street and I was explaining to him that years ago, we would not have a mural like this because it would all have been about politics and our divisions, " Ervine said.

"I told him that, for me, the mural was the epitome of the change process that is going on in Northern Ireland. The murals are changing."