The Easy Virtue star plays Neil McCormick in the movie, based on the Irish music critic's memoir, Killing Bono: I Was Bono's Doppelganger, about his experiences growing up with the U2 rocker.
Barnes was keen to meet the writer, but reveals director Nick Hamm advised him not to.
The actor tells NME.com, "I wasn't allowed to meet him. The director Nick Hamm said, 'He's so annoying in real life and so irritating that if you actually meet him and play him anything like he is in real life, the film will be unwatchable'.
"I played him as the voice he wrote the book in, and it's quite self-deprecating because he's actually quite a nice guy in real life."