ASM duties vary a lot in opera depending on the size of the company (just like in theater), but here's a quick idea of what an ASM might encounter on a typical opera production:
- ASMs participate in pre-show planning, where they create their book, generate paperwork, and ready the rehearsal hall. Like in theatre, we photocopy the score and create act/scene/aria tabs if you use those; we get working documents going for the prop and wardrobe run sheets, WWW, etc; we tape the floor and set up prop tables in the hall. But we also make sure the piano is in tune, set out water and lozenges, and provide music stands. When preparing the score, we also take timings (you can find discussions of opera timings in other discussions within this "other" forum), carefully including all cuts. Other rehearsal room differences in opera include: the maestro podium and music stand set-up, chorus chair set-up & nametags (if you do that), and a separate table area for the principal artists (stars), who indeed are usually handled with special care.
- Opera SMs and ASMs work on the deck, almost never from a booth. If there are two ASMs, one will be assigned to each side of the stage and will handle all aspects of that side - props, entrances/exits, attendance, etc.
- Depending again on the theatre, ASMs may or may not take blocking during a rehearsal. If there is an AD, the ASM probably won't take blocking, but will record entrances and exits, prop movement, and crew actions. Depending on the PSM, sometimes ASMs are assigned to be the liason to a specific production department. For example, one ASM will handle all props coordination and the other will handle costumes (this is a somewhat common scenario). When it comes to performance, both ASMs will deal with their sides of the stage, but during rehearsals, it's helpful to have one person who acts as the "official" liason between stage management and other departments.
- PSMs in opera call over intercom for places (usually at 5-minute warnings), and ASMs are usually checking off attendance lists for their sides of the stage. When you're coming up on a big chorus entrance, this can be quite a chore. It can also be a little tense as you search for the big star to come backstage for their dressing room - and ASMs give the assurance over headset that the diva is present. SMs in opera often actually give entrance cues individually to principals and to the chorus, so that entrances are always perfectly timed to the music.
- Because the PSM is usually stuck at the console constantly calling complex cues, ASMs are generally pretty active backstage. Whereas on tours, as centaura says, ASMs are sort of lackeys, in opera they are very valuable and work hard. Yes, you post on the call board and make coffee, but you also carry a lot of responsibility during a performance.
- ASMs must read music to work in opera. As you work in bigger opera houses, you'll find that everyone reads music, even the lighting designer and the TD. You don't need to speak a foreign language (though it can help), but you must be able to follow the score perfectly, even under the pressure of a complex scene change. ASMs in AGMA houses will be on AGMA contracts. It is also helpful to be familiar with other union rules, as your stagehands will be IATSE, your musicians AFM, and chorus/principals/directors will be AGMA.
Again, all this varies a lot depending on the company and the people involved. In my experience, I've found these to be generally standard practice.