There are several methods in which specific dream characters, for example well known people in real life or famous figures, can be summoned into a dream. The main techniques are listed below:
Walking into a room, and expecting the character to be there.
"Scene transition", in which the dream(er) is radically shifted to appear in a setting where the desired character "is" located in the context of a dream.
Some have been succesful in dialing an arbitrary number in a cell phone and expecting it to be the number of the person they are trying to reach. They can then ask the dream character to come and join them.
By concentrating on the intent of summoning the character. This technique is can often be used for most other objects, besides characters.