In addition to being the opera's librarian, I'm also one of the company's supertitle operators.
Supertitles -- known also as supertext, Surtitles (a term trademarked by Canadian Opera Company), surtext, and probably a host of other names -- are the English translations projected above the stage during every opera performance. Canadian Opera Company premiered the technology in 1983, in a production of Elektra; New York City Opera was the first American company to use them (in the same year). Portland Opera was the second; titles were introduced to our audiences in 1984.
If you've ever been to the opera, you've of course seen supertitles in action, but if you're new to the art form -- first, welcome! -- supertitles are similar to film subtitles, in that they project a translation of the sung text in real time, so that what you're reading and what you're hearing are happening (roughly) at the same time. The supertitle operator follows a piano score, with the cues for the supertitles already marked in; the text for the titles you see in performance is written far ahead of time and settled in rehearsal. The text is typically projected above the theater's proscenium, although in some houses, most notably the Met, the titles appear on the seatback in front of you. (The Met's system is called, aptly, Met Titles).
It used to be the case that supertitles were run on physical slide projectors, which required hundreds of slides per show. Because each slide carousel only holds a set number of slides, often the supertitle operator would have to change carousels in the middle of the show, which I imagine must have been petrifying. Also, if anything changed artistically in the show, the slides would have to be altered -- a lengthy and expensive task. These days, the titles are operated via computer; at the opera we use a Mac slideshow software called Keynote, and many other companies use PowerPoint.
I love running supertext. The supertext computer is operated from the lighting booth above the Keller's second balcony, which almost feels like an eyrie. (In the Newmark, the booth is behind the last row in the uppermost balcony, shielded by a two-way mirror, which is fun for the operator because you can watch the audience VERY up-close). I jokingly call the booth "my safe place," since I'm so far from the stage and action. Titles are especially fun when I really like a show, since I'm forced to follow the score and I'm at liberty to bop around and mouth along. I could sing along too but I'd probably drive the booth stagehands crazy. The downside to running shows I love is that I never get to watch them in the theater, since I have my head buried in the score most of the time. (Running titles during shows you don't like is awful. You're stuck listening even if you'd rather be backstage, eating cookies.)
Running the titles is a funny job. Unlike many companies, our supertitles are run by one person, not two. (Typically there is a supertitle operator, and a separate person to cue the operator). So I'm the person who's reading the score as well as the person who's pushing the button. (This is what I call the job: "pushing the button.") In order to time a title just right, you have to take a certain leap of faith: You have to believe that the singer who's about to sing the line you're about to run is actually going to sing that line. At the right time. Because you have to press the button just before the entrance in order to hit it and not be late.
The other danger is trigger finger. It's so easy to push that button the minute somebody moves to the next phrase, even if the title isn't supposed to change! There's no going back once you've advanced the slide, so then you just have to sit there and squirm until the timing lines up again. The squirming sucks. There's sometimes cursing involved.
(As a side note, one of the scariest things ever is when a singer suddenly forgets their line and sings something totally different after you've already hit the button. There is a lot of squirming involved in that scenario too, and I always feel personally guilty that I've tipped off the audience to the gaffe.)
Running titles has given me an opportunity to learn the libretto in a way I wouldn't have if I'd only sat and watched every night. It also helps my language skills. Listening and reading along in Italian, with the translation pencilled in above, does help a little in learning the language. Running titles also means I'm participating in real-time with the show, something I never get to do as librarian. It's particularly gratifying when you reach a funny title; I always have a moment when I think, "They're going to laugh!" and then they do!
A word about the actual text: We get a lot of feedback from audience members who know enough of the opera or the original language to know when a title doesn't exactly match the words being sung. Those people wonder why we chose to phrase something a certain way, or why we chose to show a slide when we did, or why we left something out. The translation of a libretto into bite-sized title slides is a tricky business. Typically supertitles are artistic interpretations of the original text, rather than word-for-word transcriptions. Sometimes this is done to help the opera reach modern audiences more effectively; sometimes it's done so the libretto makes the director's interpretation of the work clearer. Also, each slide can only contain so many characters, and often we will have to condense the translation to fit into a slide. You can only read so fast, and there's nothing more distracting than title slides flashing by too quickly to read. (You may have experienced this same frustration while watching foreign movies).
As for titles that are omitted: most of the time we omit text because it's already been repeated A HUNDRED TIMES. Seriously. There are only so many times you need to flash "I'll love you till the end of time" or whatever before everybody gets the point.
More info on supertitles:
What are Supertitles?
Supertitles for rent
The original COC Surtitles