Keira Knightley and Matthew Rhys star in The Edge of Love
Matthew Rhys "freaked out" when told he was to play Dylan Thomas in the first cinema film on the poet.
The Edge of Love director John Maybury said other "bigger" box office names were being considered for the role.
But Rhys and fellow US-based Welsh star Ioan Gruffudd sent an audition tape in and it was Rhys who triumphed.
But Maybury admitted at the Hay Festival in Powys: "What I did not think about was how important it was for the Welsh to play Thomas."
He said: "No-one has played Thomas in the cinema before".
The film which stars Sienna Miller as Caitlin, Thomas' wife, and Keira Knightley as Vera Killick, his childhood sweetheart, is premiered not in Wales but at the Edinburgh Film Festival on 18 June, and it goes on general release soon after.
The largely Welsh audience was shown some clips from the movie as consolation, but Maybury promised his next movie - whatever it is - will be premiered in Wales.
Maybury said he had originally wanted Parent Trap actress Lindsay Lohan to play Thomas's wife, but he was unable to get her out of California to Wales for "insurance reasons".
Instead, Miller an "excellent actress" and friend who came "ready made" for the role of Dylan's Irish wife, stepped in three weeks before filming began.
Maybury said Miller's career so far had been overshadowed by harassment from the tabloid press, "which isn't of her own making".
He said: "I hope that what this film is going to reveal is what an extraordinary actor she is."
The low budget "wartime" movie, shot in part in London and at New Quay in Ceredigion where the Thomases and Vera Killick lived for a while, used an abandoned Betws-y-Coed slate mine to double for Greece.
The director had spotted the mine some years before and everybody else involved thought he was "insane" for suggesting the location for shooting World War II scenes involving Killick's husband William.
But Maybury, who dismissed the suggestion of using computer generated imagery to evoke the brutality of war in the film, said the borrowed Welsh setting was as "authentic as it needs to be".
The director also revealed that when he was sent the screenplay for The Edge of Love it contained very little of Dylan Thomas' poetry.
"The first thing I did was say that if we are doing a film about Dylan Thomas I think we must put some poetry in there," he told the audience.
He said the poet's work had a "strangely cinematic quality" which lent a "beautiful refracted quality in its language".
While the film had access to some of the poems, Maybury said Rolling Stones star Mick Jagger also owns "quite a lot".
The film had the full co-operation of Dylan and Caitlin's daughter, Aeronwy, who Maybury said had been "incredibly generous in her absence."
He thought it was a "terrible burden" to be the child of someone like the poet, who was "not just an important artist but a figurehead for an entire nation to some degree".