I think that the hardest thing to write believably is romantic dialog.
Sure, it might be hard to write accurate scientific dialog, too, but it can be done; or you can settle for scientific dialog that sounds accurate and will annoy terribly the approximately 0.037% of your readers who happen to be actual scientists.
And the same thing might be true if you're putting words in the mouth of a fictional doctor, lawyer, political activist, police officer, and so on--writing their conversations believably means listening to groups of them talk, and doing other research into their fields of expertise.
But none of this is even remotely like the problem of writing a scene of romance, because all such scenes have this instinctive tendency to fall terribly flat. The writer of fiction has the worst time of it, because at least the screenwriter can count on several important things happening when his characters say the actual words of love to each other: a swell of heartrending music, a clever camera angle, a close-up on the actors' intent faces, and, usually, the resolution of a major plot point at that moment; all of these tend to drown out the commonplace nature of the actual words used.
Because the words are, "I love you." And they're so common as to be unremarkable.
And if they're expressed just like that, bald and naked on the printed page, they're nearly always going to seem trite and contrived and even a little ridiculous.
So it's no wonder that love has always been expressed in exalted language. The sonnets of Elizabeth Barrett Browning and the sonnets of Shakespeare, the poems of Byron and the verses of Keats, the lyrics of love-song writers of every generation, have fought to capture in some exquisite turn of phrase just what love is, just how deep the feeling and how constant the reality and how glad the joy of it is. And sometimes the best of them have almost succeeded.
At some point in what I hope will be the very near future the ICEL will approve a new translation of the Mass in English. There's been a lot of discussion of the proposed new translation in the Catholic blogosphere, particularly in light of this article by Bishop Donald Trautman. Bishop Trautman doesn't like the proposed new translation, and in this article he worries quite a lot about poor John and Mary Catholic, who will be hopelessly confused and even--dare I say it?--marginalized by such words as: ineffably, inviolate, suffused, unvanquished, consubstantial, incarnate, dew, sullied, unfeigned, gibbet, wrought, and thwart (all but two of which were recognized by a computer, which is a lot less intelligent than John and Mary Catholic). The bishop even issues a "Call to Action," if you'll forgive the phrase, in the hopes that Catholics who think like he does will storm the Vatican demanding an end to polysyllabism and the kind of words that are fraught with religious and biblical significance. (Is it still okay to use "fraught?" Or would John and Mary Catholic only understand "loaded?" Hmm. Can I say that my baked potato was fraught with bacon and cheddar cheese?)
I'm not trying to attack Bishop Trautman here, but I think, with all due respect, that he may be missing the point of attempting to rewrite the Mass in English in the first place.
The words of sacred ritual should be, well, sacred. They point to higher realities, to things sanctified and set apart, and ultimately, to the worship of God.
Chalice, not cup. Paten, not plate. Incense, not holy smoke.
Our words should lift us up, from the moment the Holy Sacrifice begins until we chant the final Amen. They should be, whenever possible, words that signify the great mystery we've entered into, the truly transcendent aspect of what we have gathered to do, and to become, and to be.
Because in the final analysis, the Mass is the ultimate romantic dialog, the words of love spoken by God to His beloved creatures, whom He was willing to, and did, suffer and die for. And words of love are spoken in reply by those gathered in humble worship, who know how far they fall short, and how inadequate are all their best and grandest words, to respond to that ineffable and incarnate Love, Who unvanquished by the death He endured for our sins, wrought upon the gibbet our eternal salvation, His Sacred Heart suffused with love for us as He, unsullied, thwarted the power of death once and for all.