It is not often, however, that you have only one character to concentrate on. Things get more complicated as more characters are involved.
Example: A man is sitting on a seat. A girl enters frame – she walks up to the man. She says ‘Hello’. The man replies likewise. He gets up and they exit.
That could hardly be simpler. But endless variations of that simple scene are possible.
Does the girl stop before she speaks, as she speaks, or not at all?
Where does she stop?
When does the man get up?
Does he speak from the seat, on the rise, or standing?
What is the relationship of the man and the girl as she speaks/as he speaks/as he rises?
And so on.
If that simple scene were shot from two angles with both characters in shot in each angle, the continuity has to match exactly on each angle.
On each angle the action in relation to the dialogue must match exactly, otherwise the shots 1, 2, 3 will not cut together.