|Write from the heart|
It's possible for a writer to have factual knowledge of her subject, but not intimate knowledge. This doesn't mean writers must experience all we write about, although the most meaningful stories have a kernel of truth. It does mean the writer must have a feel for the subject, a passion for the work and a personal sense of the characters that can't be achieved through research alone. Such knowledge takes a willingness to spend quality time with the story and its inhabitants on a regular basis, daily, if possible. Only time, and the trial and error of revision, can create the kind of knowledge our mentors really meant when they said, write what you know.
For more on this topic, see this supporting point: “Write What You Know” – The Most Misunderstood Piece of Good Advice, Ever.
And this counterpoint: Don’t Write What You Know.