The Usual Suspects star died in January (11) after a long illness, just months after completing work on the new comedy, which tells the story of two brothers who fail to crack the music industry and watch in horror as their school friends rise to fame as U2.
The film's director Nick Hamm reveals Postlethwaite was an inspiration to the rest of the cast. He says, "He was one of the greats, and to have him on your set was just such an amazing honour.
"The last speech that he has in the film is about fame, what fame can do to you and how fame can both destroy you and make you. I think coming out of the mouth of somebody like Pete Postlethwaite, that has a resonance it doesn't have with other actors."
The film also stars Ben Barnes and Irish actor Robert Sheehan.