'Begin Again' director John Carey worries fans will think he's ''sold out''.

The Irish director shot to fame following the surprising success of his modestly-budgeted musical 'Once' - which earned an Oscar nomination and has been turned into a lauded stage musical - and he admits he is wary of fans considering his follow-up movie a typical Hollywood production.

He said: ''Looking back, I probably had a nervous breakdown out of shock when 'Once' was so well received. Even the most cynical, hard-assed industry types were genuinely moved by it. I knew that reaction was unrepeatable and therein lay the dilemma.

''I was left biting my fingernails, wondering constantly, 'What should I do next?'

''Oh you know, 'That was so pure and beautiful and this is Keira [Knightley] and America and he's clearly sold out...' ''

Carney also revealed that, like 'Once', 'Begin Again' - which stars Knightley, Maroon 5 frontman Adam Levine and 'Avengers' star Mark Ruffalo - follows a more realistic take on the romantic comedy and doesn't necessarily have a happy ending.

He told Radio Times magazine: ''Outside of chick lit, couples don't just walk off into the sunset together - not in anything that has ever moved me, anyway. Look at 'Wuthering Heights', or 'Anna Karenina', or 'Brief Encounter' - complex, compromised love is far truer to life. Difficult relationships and unfulfilled love are what interest me as a storyteller.''