Using clues in the text, reacting to choices made by others in a scene, and interpreting notes from a director, an actor makes the choices that ultimately result in the actions we see and the dialog we hear.
Acting is a translation of assembled information into something more specific and more complex.
Although everything the actor has received through the text and the direction has been distilled into a single action, that action is still made up of all the deductions made by the actor up until that point.
No, y'know what? Acting is making a sandwich.
Let's say you have a personal chef. It's lunch time, and you'd like a sandwich, so this chef you hired is going to make one.
So you are going to supply your chef the ingredients they need and your chef will make that sandwich.
Naturally, the sandwich will be made to your order, but you are counting on this chef - whom you have proven you trust implicitly, as you hired them - to interpret your orders in a way that results in a sandwich that reaches its fullest potential while agreeing with your assumption as to what the sandwich should be.
Your chef will select the proper quantities to balance the flavors, your chef will choose the type of spread and apply it with their personal flourish, your chef will toast the bread to the crunchiness that will contrast with the texture of the rest of the ingredients while maintaining its structural integrity.
They do this, for this is their job. The result is a sandwich made carefully and deliberately, for it reflects their craft and their pride in their work.
If in their efforts an actor reaches a conclusion that does not support the scene or the work at large, a director will discuss other sensible approaches based on the text, identify an actor's motivations and reinterpret it to reach a compromise, or propose different actions to take that will influence the outcome of the scene.
This is all to direct an actor toward a conclusion that fits the director's vision and still reflects the depth of that actor's decision. That's why they're called directors and not tellers.
A director could choose instead to give the actor an emotion to play, or read the line to them in a way they would prefer it be said, with all the same inflection.
What this does is make an actor work backwards. Now that they have the conclusion they must now go back to look for the evidence.
This can result in two things. Upon being told to read a line "more loudly" with "more anger" an actor may begin to think:
"Okay. Am I angry because of a perceived sleight, or am I angry because of someone's stupidity? Am I loud in a way that I don't want to be here anymore, or am I angry in a way that I want to be listened to? Maybe I'm trying to make THEM leave..."
What you are doing is making more work. If there is nothing to suggest loudness or anger in the text, the actor will start to manufacture their own clues, resulting in a broad decision that may need to be corrected anyway.
This is the best case scenario. Otherwise, an actor might take your linereading very literally, down to the exact detail. What you are doing is setting a precedent in which an actor will do noting but what you say. This could result in an actor who makes no bold choices at all, or an actor who will ask you to vet every one of their decisions. "Should I walk over here now? Is it okay if I sit before that line? I'll sit after that line, but can I drink from the glass after I sit?"
PROTIP: Rather than telling an actor how they should feel when saying something, tell them what they should try to accomplish by saying it. "Make them regret it" or "defend yourself" or "convince them to stay" are all playable actions which can result in movements, tics, and emotions with more facets than "angry" or "upset".
So, all I'm saying is
when you give a linereading to an actor
what you are doing
is asking your chef for a sandwich
and then handing them this.
