This is called a visitation dream, or a true visitation which often entails a passed loved one visiting you in the vision. Consider it a real visitation. If from a living person, it is also considered a spiritual visit. The person surprising you has qualities that you may be lacking, and they are lending you their good energy. They may also be showing you parts of your energy, if negative, to know what to concentrate on at this time or what to heal. Comfort and familiarity. It may be your higher self using someone else, to represent parts of the self. These (self) parts visit you spiritually to add energy to your life.
See Surprise, Visitor, Relatives, Dead Relatives, Family, Party, Birthday.