To add to what @MariWrites wrote, if you’re writing a Vampire story and you’ve allowed customization for the Vampire Characters… you can also use the easier option of the “previews” command. This is coded like:
@CHARACTER previews mouth Medium Heart Vampire Fangs
And to change the mouth back [to the one that they had prior to the Vampire fangs] it’s just:
@CHARACTER unpreviews mouth Medium Heart Vampire Fangs
This method tends to be easier for some people… as it means that you don’t need to use “gains” in order to remember the chosen mouth shape. [Obviously just change the mouth shape to whichever one you want… as I just opted to add the ones that I prefer, for the example.]