This article is from the Dissociation FAQ, by Discord (email@example.com) with numerous contributions by others.
Example: A singular person may be, at heart, a quiet, introverted person.
If sie is suddenly very chatty, there will be a reason for this, such as
extreme comfort with those around, or nervousness. Or sie may be a
chatty, extroverted person, and if sie is quiet then sie feels ill or
uncomfortable. A multiple, however, may have alters who are extroverted
and alters who are painfully shy, or even mute or autistic. The differences
you see may be/often are due to different alters, not moods.
A person who has varying states of being for various situations might
well be dissociated to some extent, but not be multiple. There isn't
really a clear dividing line between simple changes of mood,
dissociation, and multiplicity. Everyone dissociates to some degree;
one interesting theory is that in a way, everyone is multiple.