As these shows are primarily visual pieces, it is important to consider how a scene will look when blocking. If your two main love interests are singing a duet, it would be better to place them in a secluded corner together rather than on opposite ends of the stage, or if you have a powerful king making a grand entrance it make sense to stage him on a raised platform while the rest of the cast looks up at him in reverence. Sight lines are simple enough to explain, they are what the audience can see. Each stage has it's own peculiarities to where the audience can see and what is considered "off stage" and it is crucial for a director to know where those lines are. Set and light design also play a role in blocking, as mentioned before if you have a grand entrance and the set incorporates a raised platform, the natural choice is to block the actor there. On the flip side, it is important to know where the lighting designer will be aiming the stage lights as an actor in the dark is an unseen actor.
There is one important member of the production staff we have not talked about yet, the stage manager. The stage manager is really the one responsible for making sure a show runs smoothly. They attend every rehearsal and take detailed notes about blocking and entrances. When a show begins its run, it is not uncommon for the director to remove themselves from the production, they may still be present and will offer notes on the performance, but at this point, the stage manager is in charge of calling entrances and making sure the blocking is carried out in the manner the director laid out.

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