I don't understand what a rehearsal swing is or why they're needed. Can you explain that to me?

The very, very short explanation is that they’re actors who join rehearsals to learn the show and don’t immediately go onboard with the cast, but are prepared to do so later if they’re needed due to injury/illness/etc. Ships typically try to minimize the number of people who need to be onboard to try and limit the number of resources required - housing, food, dressing room space, etc all has to be provided by cruise ships for their employees. Since space and resources is limited, the number of actors they bring onboard also had to be. But only being able to have one onboard cover for each role also puts them at risk of not having enough coverage. If there's just a temporary issue that will be resolved with a week or two, they usually just cancel shows until they have enough actors. Of course you know from personal experience that that happened quite a bit with the first casts coming back from Covid lockdowns - they had some Covid outbreaks and between that illness and the initial longer quarantine periods, there were quite a few shows that got cancelled. Six usually only performs 3-4 shows per week and cruise attendees pay for the cruise rather than the specific show, so it is disappointing but won't be a major issue for them. If there's a significant longer-term issue, like an actor getting a major injury that requires them to leave the ship, they're in a tough spot. That's where the rehearsal swings come in - if any are available, one of them can jump on board to replace that actor (although it's not always as straightforward as a 1-1 substitution). That rehearsal alt allows them to have someone on hand who already knows the show and can quickly join and get into the swing of things with minimal rehearsal. All around it's much easier (and much more cost-effective) than having to re-cast the role, sign the contracts, fly someone in for fittings, teach them the whole show, and have them tech it.

With Six, a rehearsal alt has only gone onboard in that capacity once. In Bliss 1.0, Alicia (Howard) had to leave the ship about halfway through the run. Bryony Duncan (B/S/H alt) took over as principal Howard. They brought in Lori McLare (rehearsal alt A/C/H) as an understudy Howard, but since she didn't know B/S they actually had Bryony continue to understudy B/S while acting as principal Howard, and Lori just understudied Howard. And to make alllll of that more complicated...Caitlin Tipping (Seymour) had to temporarily leave the show for a few weeks, but Bryony was now Howard and Amy Bridges (rehearsal B/S/P) had already joined Breakaway 1.0 as Seymour, so neither of them could replace/join. They actually wound up having Natalie Pilkington add Seymour as her fourth cover, to act as a temporary replacement. At that point they still had alt costumes so she was just able to wear her black Parr variation with no issue. (Noting here that all of this was more complicated than that, but for the sake of keeping things as condensed as possible this is the simpler version)

It does seem like it would have made the most sense for Lori to take over as principal Howard and Bryony to just continue as B/S/H (and maybe take over as S instead). Not sure why that didn't happen. Might have been a costume thing.

Anyway. That's the very shortened version of what their basic, as-advertised role is and the one time we've seen that happen! There is more complexity to it than that, and I did just write up a longer post on some of that here if you're curious! But I hope that's helpful as a Rehearsal Swings 101!

ngl i kinda find it hilarious that elèna gyasi aka new uk tour parr is tHE ONLY NEW QUEEN IN THE UK TOUR 2.0 SET DUDES I AM NOT KIDDING 9 OUT OF THE 10 QUEENS HAVE BEEN IN OTHER SIX PRODS HOLY SHIT i mean considering six's chaotic management history, we kinda saw this coming one way or another but ngl I still find this so fucking funny for some reason JFJSJFJS