Carney, best known for directing "Once", didn't seem to enjoy working with Knightley, which may be one reason why he decided to return to Ireland to film "Sing Street" and described it as "a small personal movie with no Keira Knightleys in it", reported Independent.
He said that the actress had an "entourage that followed her everywhere" on the set, which made it difficult "to get any real work done".
"So, I really wanted to work with musicians and actors that could play their instruments properly and sing and stuff like that," Carney said.
However, Carney did praise Knightley's co-stars, calling Ruffalo "a fantastic actor" and admitting he was impressed by Levine, who made his film debut in "Begin Again."
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"I think that that's what you need as an actor; you need to not be afraid to find out who you really are when the camera's rolling," the filmmaker said.
"Keira's thing is to hide who you are and I don't think you can be an actor and do that. And working with the kids (on Sing Street) and real instruments there was no hiding going on. It really was a bit of a journey of self-discovery for the actors in 'Sing Street' and that appeals to me.
"I don't want to rubbish Keira, but you know it's hard being a film actor and it requires a certain level of honesty and self-analysis that I don't think she's ready for yet and I certainly don't think she was ready for on that film."
Since her "Bend it like Beckham" days, Knightley has been nominated twice for an Academy Award, including Best Actress for "Pride and Prejudice" and Best Supporting Actress for The "Imitation Game".