It is a minor complaint amongst actors, a tiny grain of sand in the oyster of love for the audience, that the aspect of a performance that draws the most comment is often the feat of memory involved in line learning. 'Really?' thinks the actor, 'that was what you guys were focused on when I was out there getting my heart/arm/back broken? But that is just a result of physical repetition, a small tool in my technical armory. I feel so unappreciated and misunderstood'.
Anyone watching a play is bound to be impressed by the apparently smooth way in which the actors leap around the text, with iambic pentameter or Pinteresque pauses, seemingly holding the entire play in their heads. But most actors regard learning the lines as the first stage of the process; it is not until they have committed the text to memory that they can truly discover the performance they want to give. This is the ideal and the reality, for most actors, is somewhat different.
Of course, the challenge of learning the lines varies from job to job. If you are working in film, the dialogue is often sparse, it is a visual medium and it is often only in one or two scenes that the lead actors will be required to learn anything particularly epic (speeches that frequently fall into the 'I had a puppy' category as defined by David Mamet). The film actor still has to learn lines, but the trick for them will be to play with the form and make it their own, so that the audience will hardly notice that what is being spoken was scripted. This is not a question of changing the words, but of breathing the character's life into them, although famously the scriptwriter has more liberty taken with their work than the playwright. Also, the pace of filmmaking is glacially slow, two or three pages of script, on average, a day. So there is plenty of time to get familiar with the dialogue.
In television, you will often be working within a genre. There may be technical language to get grips with, such as medical or police jargon that needs to flow seamlessly. The pace is faster and if you are the lead in an ongoing series, time will have to be spent at the end of every long shooting day, learning the next day's lines. But that will be usually be it; about five to ten pages a day, unless it is a soap and then you enter a different realm. Soaps often get filmed in blocks, with colour-coded teams shooting each week and actors going from set to set for various episodes. They may have six episodes in their head at any one time, more if they have to do reshoots or pick ups. But the lines will be scene length, a few pages long. The actors do not need to learn whole episodes at a time.
Then there is theatre, where the actor actually does need to learn their entire part during the rehearsal period. Some actors like to get to grips with the lines before they start rehearsing but this is often not practical for a couple of reasons. Firstly, they may not be cast until quite late and secondly, because learning the lines completely out of context is quite odd, like memorising random items on a conveyor belt. When the movements of your character and the faces of the other actors become known, then the lines start to make sense. For this reason some actors deliberately try not to learn their part before they have thoroughly explored the play in rehearsal; they want the lines to be integral to the interpretation of the character.
So, what about the actual learning? It is exquisitely unexciting. There are some variations in technique, for example dyslexic actors will record their lines (how much easier now we are digital), and then listen to them or record the other character's lines and fill in their own according to their taste. Some actors like to write out their part by hand as they become more familiar with the words, because the physical act of writing the lines down helps to strengthen the memory.
But the main way of learning lines is plain old repetition. You get familiar with the words when you start rehearsing and then you go over them in the evening, covering your part and stumbling through, asking whoever will help to listen to you and then repeating them to yourself incessantly as you go about your day. Driving is a great time to do lines. Lines tend to make themselves known when you are sleeping as well (preferably not while at the wheel), swimming around your subconscious in a disturbing fashion. Suddenly, everything that you hear reminds you of your play and your friends and family become almost as crazy as you, while you chant bits and pieces from forthcoming attractions in a frenzy.
And then there is the learning of everyone else's lines as well. Not their entire part, more the beginning and the ends, well, you want to leave yourself some surprises when they chat away while you listen night after night for the next several months. But you'll need to know the starts and stops because basically, that's where you come in (you cannot, unfortunately, say all your lines in one go). Your colleagues don't always make it easy for you. They may take a lot longer than you to learn their own lines, (they may never quite know them) leaving you to figure out if they finished speaking or have a little bit more to say or if they're ever going to mention that bit about the wardrobe that you're supposed to ask them about next. But if there's a big pause in the middle of the scene while you wonder whose turn it is to go next, it's usually yours.
I wish it weren't so prosaic. I wish there were some cunning trick, a shortcut that the Magic Circle of Equity members revealed to the novice before their first night, or even after just for kicks. I don't think most audiences watch a play in the hope that some spectacular mishap will occur, although I have been told often enough that 'audiences love that sort of thing' when I have been responsible for some spectacular disaster. Actually, audiences rarely notice when things do go a bit wrong. Not horribly wrong like when you might be left alone on stage while your fellow actor screams obscenities into the wings before deciding to return. For instance. That, they do tend to notice. But minor mishaps, like skipping ahead an act before realising and then running through the missing pages in your head while carrying on with the play. Mostly, they don't notice that.
This is why the 'actor's nightmare' is not just a cliché. We really do wake up screaming about having to go on without knowing the lines. The feeling is so completely terrible, so sweat soakingly, heart-poundingly panic inducing, that even though there is absolutely no obvious way to do it, every time we start a job we eject large quantities of essential data, (how to file tax returns/recognise family members/tie shoelaces) and shove the whole play into our heads. It's not life saving neurosurgery, or going down a mine, but it is quite tricky. And yet, we still get ever so slightly, very respectfully, affronted when that is the only thing asked of us after a show.