The actress said that the Cheerios captain is trying to be more "genuine", but could very easily slip back into her old ways.
"She hasn't changed her stripes, so if something p*sses her off she could switch," E! quotes her as saying.
"But I think she was really sincere [about helping New Directions]... and at least in this moment, she means it."
Last year, Lynch attributed Sue's change to the loss of the character's sister in Glee's second season.
She recently admitted that the rise of her profile since playing Sue was "more exciting for other people".
Glee airs on Tuesdays at 8/7c on Fox, with the season finale scheduled for May 22. The show airs on Sky1 in the UK.