"It is she whom we are talking about."与"It is her that we are talking about."有什么区别?

"It is she whom we are talking about."与"It is her that we are talking about."有什么区别?
"It is she whom we are talking about."句中为何强调的是宾格"her"却写成了主格"she",那么,"It is she who/that we are talking about."与"It is her who we are talking about.可以说"It is me that told him the truth.