Regarding singing along: it depends on the song! The audience won't sing along if nobody else knows the song, of course, but they'll also often let the singer take centre stage for particularly dramatic or emotional songs that give a solo singer the opportunity to show off. For example, I know 'Hurricane' from Death Note: The Musical very well, but I wouldn't dream of intruding on Tem's performance of it! We'd definitely all join in on a big crowdpleaser like 'Livin' on a Prayer', though.
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Regarding singing along: it depends on the song! The audience won't sing along if nobody else knows the song, of course, but they'll also often let the singer take centre stage for particularly dramatic or emotional songs that give a solo singer the opportunity to show off. For example, I know 'Hurricane' from Death Note: The Musical very well, but I wouldn't dream of intruding on Tem's performance of it! We'd definitely all join in on a big crowdpleaser like 'Livin' on a Prayer', though.