Sienna Miller was ''speechless'' when she met Clint Eastwood.

The 33-year-old actress worked with the 84-year-old star on his latest directed project, 'American Sniper', but the blonde beauty got star-struck when she first met him because he was so ''charming''.

She said: ''I didn't meet him until I flew out to LA to start shooting. He's just the most charming, handsome, charismatic figure. I was slightly speechless.''

But Sienna admits she feared she might never get to meet the 'Million Dollar Baby' star as she thought she'd done badly in her 'American Sniper' audition.

She explained: ''Clint doesn't meet actors. He feels like if he did, he'd cast them all because he remembers how agonising that audition process is.

''He famously only casts off-tape. So I went and it was a 20-minute audition, and I left and felt bad about it but somehow it worked out.''

Fortunately, it all went well for the actress and she landed the part of the titular character Chris Kyle's (Bradley Cooper) wife Taya Kyle, and Sienna is grateful to Taya for giving her access to her private emails to help with the portrayal.

She added to Time Out magazine: ''I Skyped with her first then spent time with her when we got out [to the US].

''She kept everything because she never knew whether he would survive.

''There was so much source material, she had every email they ever wrote to each other and she gave me access to those.''