Dexter Fletcher has promised Peter Mullan will wow with his singing voice in his new musical.
The Lock, Stock And Two Smoking Barrels star is directing the 53-year-old Scottish actor and Jane Horrocks in Sunshine On Leith, a film adaptation of the stage show featuring the music of the Proclaimers.
Dexter said: "Peter Mullan sings... very well! People are going to be happily surprised. The man's a great actor but he's also a great singer, it's no surprise. He's great!"
The film follows the stories of two young soldiers, who have to re-learn how to live life after coming home to Scotland from serving in Afghanistan.
Former child star Dexter has had critical acclaim with his directorial debut Wild Bill, and said it has driven him to make more films.
He said: "It was not necessarily the plan with Wild Bill. I just wanted to make a film which I thought was a good story about some issues I was thinking about, and that critical acclaim has allowed me to move on in directing.
"So if people think I'm reasonably good at it or I've done a good job, then I'll keep doing it until they don't let me any more.
"Directing is very different. Acting is about detail and directing is about the big picture. With acting you can be quite insular, where as with directing you have to have an eye on everyone, you have to nurture, which is a good thing to do. And you have to have a vision."