How did this actor "become" the character so well?

They mean she is unique, Evanna is weird... not in a bad way but she thinks like no others think, she is open minded and okay with trying new things (Just like Looney Luna Lovegood)... Anybody can play a role but it takes a unique person/character to actually be the person, and she IS luna, her look, the way she thinks, the way she talks....etc