Quoted from a 1976 interview of Aurelia Plath by Robert Roberton, published in The Listener, Vol. 95, pp. 515-16. They're discussing The Bell Jar:
Roberton: [W]hat sort of similarity do you feel between yourself and Mrs. Greenwood in that story?
Aurelia Plath: Very little, really. As my son and I analysed it, the words uttered by Mrs. Greenwood were uttered by five different individuals in real life. The counsel Sylvia gave me to bear in mind, whenever I read anything that she wrote in the form of poetry or prose, was: 1. that there is a manipulation of experience--this is part of the creative act, of course; 2. that there is always a fusion of characters--that's very, very evident; 3. that she firmly believed that art was a rearrangement of truth--this was to make the art form more consistent than life ever is.
[The Listener was the BBC's weekly print magazine, published from 1929 to 1991.]