The message can be in many forms - dance, music, prose, or pronouncements. Most channel instruments, the humans allowing their bodies to be temporarily used, know when this has occurred, and so does the intended audience. The channeled message is different from what the human, who in most cases is familiar to the audience in some way, would have produced. Most channeling is done to produce words, the effect of which can be broad reaching as it can be translated into many languages, and shared repeatedly. Channeled dance is brief and fleeting, and prose has impact on a smaller audience in general than written works.
Channeled music is also somewhat limited, as the appreciation for music varies by culture.
Therefore most forms of channeled information are the written word. Many channels are imperfectly attuned to the entities they are putting in the driver's seat. Such a simple matter as constipation or a headache can interfere. Most definitely, the human channel's own fixed concepts can interfere, as surely as a locked steering wheel will cause a car to go in a circle. If the channel is blind the entity trying to channel cannot see what is being written, as the entity is in contact with the mind of the channel.