Slow Hands Niall Horan Download 'Slow Hands' on iTunes
Actress will star as Effie Gray
Knightley has been cast as the wife of 19th century art critic John Ruskin who left her husband for the Pre-Raphaelite painter John Everett Millais.
The love triangle has been dramatized several times in plays and in an opera.
Emma Thompson is currently shooting her own film version, called Effie.