Bowing or prostrating to anyone other than Allah (may he be exalted and praised) is shirk but does that also include what some actors do at the end of a play?
Bowing or prostrating to anyone other than Allah isn't shirk unless you intended to worship the other object. If they were worship on their own with no niyyah, you'd be worshiping the things in front of you even if you didn't want to.
Therefore, it matters on the intention. The actor, or any performer, bowing to the audience isn't committing shirk, or even a sin, as long as they're not on the intent of worshiping the audience or believe them to be gods besides Allah.