There are several 'ways in' to directing professionally, and one of the regularly recommended ones is to assist a director or company whose work you respect and want to be a part of (assuming you can get the gig). Most of the time this will be voluntary, and you probably won't even get paid expenses. But what does an assistant actually do? And why on earth *should* anyone pay them?
The fact is that the role of the AD is entirely dependent on the temperament of not only the AD themselves, the director and the company, but also the nature and size of production, and the experience that the AD can bring. It is likely that if you land a gig AD-ing, but have never done it before, you may be given less to do or say than if you have more experience.
ADs are more prevalent in large, funded companies, such as the RSC or at the National Theatre. They are particularly useful in large-scale, large-cast productions, but because of funding limitations, these are not only hard to create but also hard to get into. Also, like any partnership in the creative industry (and, yes, the relationship between the director and AD does need to be a partnership - even if it's unequal), you tend to see the same directors using the same assistants.
But what do they do?
1. Research (historic periods, literature, playwrights, prisons, torture) - quite often the AD can become the 'go-to' person for any information pertaining to the text in question. On the plus side, you quickly become an expert on one or two topics. Remember that the internet is both a friend and enemy when it comes to accuracy...
2. 'A second pair of eyes' - this is the biggest part of being an AD. To know and understand the vision of the director for the production (obtained through discussion and attending as many preliminary meetings as possible) so that you can monitor the show in rehearsal and performance and help it remain true to that first idea. This can require questioning certain decisions in rehearsal, and it's important to learn how to 'take' being shot down with an idea you may yourself think is really good - but isn't liked by the main director. An AD needs a thick skin!
3. Sometimes, if the cast is very large, particularly if it constitutes children, and the rehearsal period short, or if the director cannot attend a rehearsal, the AD will lead (parallel) rehearsals. This can be quite scary, and relies on the company accepting the AD's authority. Sadly this is not always the case, but in general casts and crew are supportive of everyone in the room. This can be one of the best parts of being an AD - you get to flex your directing muscles for once! But equally, you still have to hold true to the director's vision, so it can be truncating. In some companies (e.g. RSC), the AD is fully in charge of rehearsing the understudies - and can be involved in casting them from the available actors. However, again because of financial constraints, fewer companies are able to employ understudies.
4. Monitoring the show. Generally, once a show is open, that's the end of the director's job. They may check up on it throughout the run - but usually a maximum of once a week. As an AD, I have (so far) always been encouraged to see the shows as often as possible. This keeps a company on its toes, can provide much-needed reassurance/guidance if things aren't working for any reason, and helps keep the director in touch with the show without having to actually attend. In some ways, it also gives the AD more ownership of the show, as you can be more directly involved in note-giving than at other times in the process.
5. 'Resident'. On some productions, the AD will be attached to the show almost indefinitely. If the show transfers, and the original director is unavailable, the AD may well lead the re-rehearsal period. ADs will follow tours of the show, and can end up with a more pastoral than creative role. For long, established runs in the West End, the AD will rehearse new cast members when they join the company (classic example is Chicago), and this again can give you a stronger sense of ownership than may originally be felt in primary rehearsals. This tends to be the only time that an AD will (or should) definitely be paid for their work.
These are only a few of the things that ADs do. Sometimes, the AD just has to sit in the corner and shut up - this is certainly the stereotype. Quite often you become a PA to the director as well as AD, and in many respects you can become a 'buffer' for the director - taking enquiries from other departments to set up meetings, running the director's schedule and taking care of little errands. You are also - potentially - a social bridge to the actors. It is a useful skill for an AD to be able to 'take the temperature' of the feeling of the rehearsal room, and know how to manage it on the director's behalf or warn them in advance if trouble is brewing.
Fundamentally, I think the AD is the director's ally. The person they can turn to and say 'this is shit, isn't it?' or 'this is great, isn't it?' and receive positive, constructive feedback. You are utterly on the director's side, but - and this is important - not just a 'yes' man.
And if you can do that - lay aside your own ego in support of a production - and enjoy it (and it can be immensely rewarding), then AD-ing may be the way forward for you.