Diagramming the beats of a scene is an art, not a science.  There is room for reasonable people to disagree.  A change in your scene partner’s emotion or tactic doesn’t necessarily mean anything changes for you.  Some actors will identify the smallest nuances, while others will take a slightly broader view.  (Taking the more detailed approach isn’t necessarily “better”; it is entirely possible to identify fewer “formal” beats but nevertheless play the subtleties within them.  Going the detailed route can also become tedious by the second act if you have a major role in the play!)

I typically mark the beats in my script in pencil, because as I go through the rehearsal process, I may find that I change my mind about a beat’s placement.  Even if I give them no conscious thought after I mark them, the slashes serve as a subsconscious reminder of where there is a bend or turn in the road.

Here’s a monologue by Dora Strang, from Equus, by Peter Shaffer.

Look, Doctor:  you don’t have to live with this.  Alan is one patient to you:  one out of many.  He’s my son.  /  I lie awake every night thinking about it.  Frank lies there beside me.  I can hear him.  Neither of us sleeps all night. /  You come to us and say, who forbids television?  Who does what behind whose back? – as if we’re criminals.  /  Let me tell you something.  We’re not criminals.  We’ve done nothing wrong.  We loved Alan.  We gave him the best love we could.  /  All right, we quarrel sometimes – all parents quarrel – we always make it up.  /  My husband is a good man.  He’s an upright man, religion or no religion.  He cares for his home, for the world, and for his boy.  Alan had love and care and treats, and as much fun as any boy in the world.  /  I know about loveless homes:  I was a teacher.  Our home wasn’t loveless.  I know about privacy too – not invading a child’s privacy.  /  All right, Frank may be at fault there – he digs into him too much – but nothing in excess.  He’s not a bully. . . /  No, doctor.  Whatever’s happened has happened because of Alan.  Alan is himself.  Every soul is itself.  If you added up everything we ever did to him, from his first day on earth to this, you wouldn’t find why he did this terrible thing – because that’s him; not just all of our things added up.  /  Do you understand what I’m saying?  I want you to understand, because I lie awake and awake thinking it out, and I want you to know that I deny it absolutely what he’s doing now, staring at me, attacking me for what he’s done, for what he is! /  You’ve got your words, and I’ve got mine.  You call it a complex, I suppose.  But if you knew God, Doctor, you would know about the Devil.  You’d know the Devil isn’t made by what mummy says and daddy says.  The Devil’s there.  It’s an old-fashioned word, but a true thing . . . /  I’ll go.  What I did in there was inexcusable.  I only know he was my little Alan, and then the Devil came.

The slashes mark the start/end of the beats I’ve chosen on my first pass.  As I say, there is no right or wrong here.  You might want to have “We loved Alan” begin a new beat.  You might want to put all the lines about “privacy” into one beat, beginning with “I know about privacy too” and ending with “He’s not a bully.”  You might want to include “Do you understand what I’m saying?” in the beat that precedes it, and start the next beat with “I want you to understand.”

Wherever you choose to place your slashes will impact how you deliver the monologue.  Make the changes I’ve proposed in the preceding paragraph, and it will change how you say the lines.  In subtle ways, perhaps, but there will be a distinct change in what is going on inside of your character.

I won’t go through the piece to describe the changes from one beat to the next; for the moment, I prefer to let you understand them in your own way.  I’ll give you a detailed analysis of it when we get to talking about playing verbs.  But if my choices confuse you in any way and you’d like some clarity now, just let me know, and I’ll be happy to explain my logic.

See Part I here.  See Part III here.  Or just skip to Why Playing Verbs is (Ultimately) Easier than Acting Emotions here, which is where you can read about the changes from one beat to the next (which uses this monologue as an example).  See Playing the Verbs Part II here.