Some calls are most definitely negotiated into the actor's contract.
"Last Star Bow" is a term for a solo call and the last one onstage. No one else is onstage when the star's entrance is made for their bow. Think most curtain calls for Rose at the end of Gypsy.
Solo Bow is when there company stays onstage and the star enters, bows, and then the company leaves stage (or the grand drape comes in, they leave and then it goes up again) so the star can have a solo bow. Then the company returns.
Yes, the bows are set by the director, but they are often mandated by agents doing their job. Some actors get solo bows (and featured billing) instead of more money.
What happens at the end of Wicked? Both actresses enter at the exact same time (or at least Idina and Kristen did), both walked down center and bowed at the same time. No one came onstage first, neither bowed last and neither took an extra bow. That's agentry. Also tact on management's part.
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