1 Mercutio: The next thing we need to do is find actors to star in our film.
2 Shakespeare: I can act!
3 Shakespeare: Alas, poor Yorick! I knew him, Mercutio: a fellow of infinite jest, of most excellent fancy: he hath borne me on his back a thousand times; and now, how abhorred in my imagination it is!
4 Mercutio: Don't give up your day job.

Acting is one of those things that seems easy - almost so easy that anyone could do it. But the more you learn about acting, and also the technical things that go into film production, the more you realise how difficult it is, until you eventually reach a point where you wonder how anyone can actually do it. It's not just memorising lines; it's way more than that.

From my point of view as a photographic imaging professional, I've done some research into the technical side of film and video production. Film and TV actors don't just have to memorise lines and deliver a believable performance, they have to do it with all of the technical limitations of film equipment in mind. A big one is focus.

Traditional motion picture cameras, and virtually all of them still in use today for major film production, don't have autofocus like your digital camera or phone. They're all focused manually. There's actually a person on film set production crews who has the sole job of focusing the cameras. This person is called the focus puller. And they're not doing what you might expect at first thought. The cameras roll, the director yells, "Action!" and the focus puller adjusts the focus as things happen so everything is nice and sharp in focus.

Nope.

It doesn't work that way. That's much too difficult for a human being to do in real time. It's essentially impossible. You'd end up with the focus all over the place and your lovely film all blurry.

A focus puller works intimately with the director and the actors, setting up (or "blocking") a scene so that all members of the cast and crew know what action is going to happen, and most importantly for the focusing of the cameras, where the actors will be and who the director wants to be the focus of attention at all times. In a dialogue, attention will often flick back and forth between two or more actors. If they're not all precisely the same distance from the camera, then the focus will also have to flick back and forth between them. You don't even notice this when it's done right - but if it was wrong the scene would look like a mess. It's even harder with action scenes where actors are moving. The focus puller needs to know exactly where they will be at each moment of filming, so they can anticipate the move the focus manually to track them. They can't afford to react a fraction of a second behind - they need to know in advance.

Typically the actors will have "marks" - usually bits of coloured tape placed on the ground to indicate where they need to be at each beat of the scene. The focus puller uses a tape measure (or maybe a laser distance tool these days) to measure precisely the distance form the camera to each mark in the scene, and records the details. Then the focus puller will often make physical marks on the camera focus dial, with a pencil or tape, corresponding to those positions. This allows the puller to adjust focus rapidly (by hand) to match an actor's position whenever they move to a new mark. And the focus puller has to get this exactly right. If an actor's face ends up slightly blurred in a shot, there's nothing you can do but reshoot the scene.

And to help the focus puller get this right, the actors have to do their job exactly right. They have to hit their marks precisely, every single take. They have to know where the marks are, and land exactly on them after walking or running or fighting, or whatever they're doing - and they have to do it without obviously looking for the marks. Because you'd notice if an actor was always glancing at the ground to see where they need to be standing.

And in a dialogue the actors aren't allowed to move around too much, because that could cause them to drift out of focus. They need to keep that mark position as they deliver their lines, without bobbing their heads back and forth too much or whatever. And they need to do this so that it looks natural and believable. (For actual action scenes, things are a bit more forgiving. You can set the camera aperture to enlarge the depth of field, so that a bit of movement won't throw things too noticeably out of focus. But for close up dialogue, the depth of field is usually very narrow, and a bit of movement can make the actor look blurry.)

This is just one component of the stuff that actors need to know and incorporate into their work. As I say, the more you learn about film production, the more you realise how technical and demanding it is. I'm sure there are even more things that actors have to deal with. Whatever it is, acting is not an easy job.