Original: Daniel Radcliffe, Rupert Grint and Emma Watson, who have played and will play Harry Potter, Ron Weasley and Hermione Granger in all the films, were virtually unknown when they were cast.
Simplified: Not many people knew Daniel Radcliffe, Rupert Grint and Emma Watson, who play Harry Potter, Ron Weasley and Hermione Granger in all the films, when they were cast.