People usually act the song out to some extent while singing it, but the point here is to act out all the lyrics, trying not to miss any words/phrases. It's silent singing. There could be many different versions of the game, here are a few I can think of:
A person chooses a song the lyrics of which he/she knows by hard (just like with karaoke). He/she can sing as well while acting or only act. The record that is playing can be with vocals or without (a karaoke version), but if the person is only acting, then the record should contain vocals.
One person sings a karaoke version of the song while another one acts out the lyrics.
Other people choose the song for you, you might not know the lyrics at all or know them only to some extent (this can be discussed within the group with the person who will be acting).
Different people act out the same song one by one, the results are compared.
Different people act out the same song at once.
A person acts out the lyrics of a karaoke version of some song that only he/she knows and the spectators try to guess what the song was about, its title, etc.
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