In my experience, the answers are all of the above.
Some kids just know that they belong on the stage singing and dancing and acting. Other kids are young enough that they are willing to do as their parents suggest (and end up having fun despite what they thought was going to be hard or embarrassing). Or they just go along with what their older siblings are doing.
In the following clip you can see examples of all of theses situations.
Indeed you can see kids who are timid to try something for the very first time. You also see the ones who would rather their parents had signed them up for soccer camp. But then there are the ones who enjoy performing and just know that someday they will be a star!
What makes me happy is that they all tried to sing in tune, learn all the words, remember the choreography, and help each other create a fun show.
I'm so proud of my little neighborhood group of thespians!