My earliest memory of a backstage space is a bunch of us crouched behind an overturned table in a school library, where I had created a play about something or other – The Wizard Of Oz, I think. I must have been about eight. The invisible but ever so tangible line between backstage and on stage can be terrifying to cross, especially when you have to immediately say something like “To be or not to be”, or “Now is the winter of our discontent.” It is the nearest I come in this life, I sometimes imagine, to being born and dying.

To be backstage during a play is to be in a twofold world of secrecy and revelation. It is to live in two periods at once: the time of your own life and the time of the character you are playing. There is a similar feeling in standing next to a river, a bonfire, on a platform when a fast train is approaching, or, I imagine, beside an open door on an aeroplane.

Backstage, I always have one ear to the house, judging the energy of the audience from their response to other scenes, enjoying the innovations and discoveries of my fellow actors, and privately harnessing the aspects of myself, the thoughts and actions, that are appropriate for my character. In Twelfth Night, I was playing the Countess Olivia, a grieving aristocrat who has inherited control of her house and its difficult occupants after the death of her beloved father and brother.

Just as there is a certain ritual to the action on stage, so there is backstage. The quiet preparation for an entrance, the quick costume change, the motivated exit that deflates rapidly in the dark, the jubilant energy you get with an expressive audience, the relaxed energy of actors who have finished their part and are waiting for the final call, the regular absence from the stage that allows for reading, correspondence or games of ping pong: all the backstage after-and-before shadows exactly what happens on stage; it is both a private and a social space.

The experience is at its best during an ensemble play such as Twelfth Night. Every company is an ensemble, but some plays are not so evenly shared between as many players. In this production, the ensemble was enriched because many of us had worked together regularly over a number of years. Directed by Tim Carroll, we used as many of the known practices of Shakespeare’s day as we could, trying not to do anything they couldn’t have. Hence we were an all-male company, with three of us playing female roles. We used authentic makeup, including silk wigs, and performed in a shared candlelight on a wooden stage: no scenery, minimal props and furniture.

The Belasco theatre, where these pictures were taken, is an old Broadway house with a large stage. We had room in the wings for two oak “standings”, an old Elizabethan term for raised platforms on which an audience could stand or sit. All our entrances were via two doors in a reconstructed oak screen, which also provided a high gallery for our musicians.

But we didn’t choose the Belasco for its size; we chose it for the great space underneath the stage, where those who wished could dress together, and where we could have a ping pong table and post-show social club (sometimes during the show as well). I had heard that Houdini had created this deep space under the Belasco, to enable an elephant to disappear by dropping through a trapdoor into a tank of water. My friends there denied this, but there was certainly room. It was so cold that winter, we spent many a memorable late evening in our Houdini cave, after the play had finished, playing ping pong and table football, entertaining guests. At our Christmas party, Stephen Fry donned his Santa Claus suit and passed out the gifts.

A good theatre feels like a great ship. Front of house, its stalls, circle and balcony, are like three great sails filled each day with the imaginative life of an audience. Backstage feels like life below decks. Enjoying a run of full houses – well, there’s nothing like it, in all the different jobs I have been lucky enough to experience, perhaps nothing like it in life. It is like sailing a fast ship on a sunny, windy day.

Over the years, we must have performed Twelfth Night at least 300 times in different formations, with actors leaving and returning, or moving to different parts. Some have grown too old to play their part, such as our first Viola, Eddie Redmayne. Some have passed, bless them. In New York, some, such as Stephen Fry, were joining us for the first time. It was for me the most mature, humorous and accomplished ensemble I have worked with; I trust we will perform a few more before we are done.

• Twelfth Night by Mary McCartney, with a foreword by Mark Rylance, is published on 17 November at £24.99.