They made a surprise appearance to say they would take part in a question-and-answer session after the film. It is based on Eric Lomax's book of the same name about his time as a POW working as a slave labourer on the Thai-Burma railway in the Second World War. The actors play Lomax at different stages of his life.
Lomax's widow Patti, 75, was also there with scriptwriters Frank Cottrell Boyce and Andy Paterson, and producer Bill Curbishley.
Originally from Edinburgh, Lomax settled in Berwick with Patti, who is played by Nicole Kidman. The film is being screened in the small cinema in the town for a week before its national release on January 10.
Unusually, both actors were able to meet the man they were to portray in the film, which tells of his harrowing ordeal at the hands of the Japanese during the Second World War.
During secret visits to Berwick, they forged a firm friendship with the couple and continued to support Patti after Lomax's death in October 2012 at the age of 93.
Both actors said on Friday that making the film was unlike anything else they had done.
Said Firth: "It has been an immense experience compared with anything I have ever done and has not ended with the shoot as it is still going on here and now. When we went to visit Eric and Patti, we entered the story and became part of it in a way.
"It is not just what happened to him in 1942 and beyond - it is about the telling of these things and it is a testament to storytelling itself.
"The fact that he came back and published that book is incredibly important and that's why we are all here. It is a very big experience and not just like making another film. This became very, very personal for all of us - it went very deep for everyone concerned."
Irvine said: "It will stick with me for a very long time."
The film shows how Lomax suffered post-traumatic stress after the war and how, with the help of Patti, he was eventually reconciled with the Japanese interrogator who had tortured him.
Said Patti: "I think it is a marvellous film. It catches the feelings of confusion and fright and wondering what on earth is going on when someone is having problems they cannot express to you."