My only experience of acting is in theatre, and only at the school/university level. But I think theatre professionals would probably agree with the following description of the job, at least for conventional plays that don’t include improvisation or audience participation: in rehearsal you try all sorts of things, your own ideas as well as the director’s, but by opening night you need to settle on the best possible version of your performance, and then repeat that every time.

Film seems to me to be utterly different. Not just in the mechanics of acting – the subtle expressions and natural voices, rather than the bigger faces and projected speeches that theatre requires. It’s different because it’s in tiny pieces, kaleidoscopic rather than whole. Your job is to give the director a wealth of material – footage – from which they can put together a film. Your attempt to inhabit your character, or to build up any sort of emotional momentum, is constantly interrupted.  Not just by the cry of ‘Cut’, but by the pragmatic need to retouch makeup or a strand of hair, adjust the lights, because a fire truck was roaring by or your foot was slightly too far to the left or the mouse won’t come out from under the refrigerator or your child star is legally required to have a play break now.

Some actors famously cope with this by shielding themselves from distraction, staying in character, withdrawing to their trailer, or enforcing clauses in their contracts that specify no one except the director and producer is allowed to address them or look them in the eye. But most seem to adopt a workmanlike attitude, joking away as they have their mics or hems adjusted, then summoning up emotion – even tears – as needed. Either way, it’s a demanding job.

Because most films are shot out of chronological sequence – with the order of scenes determined by practical considerations of efficiency and cost – a director may not yet know how they want the scene they’re shooting to feel; that all depends on how it will fit in before and after other scenes in the edit.  So what an actor may be asked to offer in different takes is the widest possible range, not just of emotional temperature – say, furious to neutral to loving – but of other factors too: run the lines fast or slow, very naturalistic or heightened, stick to the script or improvise… Some of the ways the director will ask you to play a scene will probably feel dead wrong to you, but your job is to try anything, to keep pouring energy and intelligence into actions and lines that you may have done twenty times already. ROOM was a very different experience because Lenny had a strong sense of how he wanted each scene to work, and filming it mostly in sequence allowed the actors to share that sense of clarity about the story they were telling.

The experience of creating your performance in these jigsaw pieces may be bitty – almost intolerably so, to some actors – but hopefully, after editing, to the audience in the cinema your performance will look like a seamless thing, as if you’re living it out before their eyes, right here, right now.