The pair ( Mullan and Henshall) first met nine years ago when they worked together at Glasgow's Tron Theatre in Crow.
At the start of the run, their play was struggling, but the two actors bonded as they tried to find ways of turning the show around.
Mullan recalls: "We died each night for four nights and you really get to know someone when you bomb together.
"We had mates who couldn't look us in the eye.
"But when we finally cracked it, I knew that Douglas was a guy I could trust."