So, the poet Robert Frost said, "no tears in the writer, no tears in the reader". This, I think, we usually take as being about writers having to be willing to feel what they want their readers to feel. Indeed, although Wordsworth, in his Preface to the Lyrical Ballads, famously describes poetry's origin as "emotion recollected in tranquillity", he goes on to say
the emotion is contemplated till, by a species of reaction, the tranquillity gradually disappears, and an emotion, kindred to that which was before the subject of contemplation, is gradually produced, and does itself actually exist in the mind.
In other words, you re-connect with the memory, thought, or sensation which you are no longer experiencing - and in doing so you will be affected again by something like those feelings. Wordsworth and Frost are, by implication, also talking about authenticty: that if you want to affect the reader strongly, what you write has to have made that connection, in the sense that it has to come from some kind of honest place in yourself.
Perhaps that's obvious if - as people tend to assume, often wrongly - the poet's poem is rooted their own experience. But I think this re-connection is also key to writing well when the project is fiction, or something else which ranges more widely, or is about something that you don't have direct experience of. If the job is using your imagination to enter into an alien (in the technical sense) experience then in some way you'll need to find a connection with an analogous part of yourself, and then that "kindred emotion" will be awakened.
But I think it's easy to take Frost's injunction in the wrong sense: to assume it means that the uncensored outpouring of emotion is what's needed to make writing that will draw those emotions out of the reader - but that would be a serious misunderstanding. All actors know that if you totally break down on stage, no one in the audience will be able to hear a word, you'll mess up the staging, and wreck the story the play is trying to tell: the art is in finding that authenticity, but keeping part of yourself outside it and in control. And although I'm not the only writer who has found tears in their eyes when drafting a big scene - especially at the end of a book - we're still quite capable of writing. Through our tears we go on tweaking the words, keeping an eye on the basics of pace, tone, psychic distance and showing-vs-telling, getting the punctuation right, spelling correctly and making sure we've dropped the right clues and held tight to the ones we don't want to drop yet.
The fact that there's more to do in revision doesn't negate that dual experience, nor does it mean we are shallow hypocrites, just that we have developed a certain human faculty to a greater degree than normal. Actors are both inside their part and outside it, like jockeys, whose job is to awaken the flight response in the horse, and then control it. A soldier, too must learn to awaken the fight-or-flight response in herself or himself and the platoon, and then control it: the control is as necessary to the job as the innate response. Presumably Wordsworth's necessary "tranquillity" is about starting from that place of control, before beginning to re-awaken the emotion. The fact that you can operate in that divided way doesn't mean that you've lost touch with authenticity and honesty; it's just part of the necessary schizophrenia of the writer. (I'd be interested to know if keeping these separate processes going simultaneously is something we share with our fellow great apes, or something distinct in homo sapiens.)
And there's another way that Frost's point, important though it is, can mislead writers and artists. Remember Andrew Stanton's "Law of two-plus-two"? He explains that the audience can't help doing the maths to make Four, and that they positively want to, provided they don't realise they're doing it. It is not a coincidence that the original title for Stanton's TED talk was "Storytelling is Joke Telling": we know that explaining a joke kills it, but actors know more: that if you start laughing at your own comedy not only will you wreck the pace, rhythm and tone of the production, and find that your speeches go unheard, but, worse, the joke dies. It's the audience who must make the joke - and do the laughing. Even standup comedians only laugh once the audience has started. So the Fours the audience make for themselves - the tears, the laughter, the understanding - will always be more powerful than the Fours you write out for them, en clair on the page: it's their own Four.
One more thought: remember choirboy syndrome? How agonisingly long you can go on giggling inside, when you mustn't laugh out loud? And, too, recall how much more heartbreaking it is to see someone in terrible circumstances trying not to cry, than when they do cry. Denying your audience the explicit Four, as it were, can work even more powerfully if you also deny your characters the relief of completing the emotional equation by reaching their Four ... at least, not yet. Probably soon, although fortunately - but unfortunately ... maybe not yet ... now, just a little ... but shhhh! someone's coming ...