They made a surprise appearance on Friday in Berwick-upon-Tweed and answered questions after the screening.
The film is based on Eric Lomax's book of the same name about his time as a Second World War POW on the Thai-Burma railway. The two actors play Lomax at different stages of his life.
Lomax lived in Berwick with his wife Patti, who is played by Nicole Kidman in the film. The widow was at the screening of the film, which runs for one week before a national release on January 10.
Firth said: "The fact that he came back and published that book is incredibly important and that's why we are all here."
The actor, who visited Lomax before his death in 2012, added: "This became very personal for all of us - it went very deep for everyone concerned."