At the lowest level, there is a distinctive feeling that another form of consciousness is internally present alongside that of one's usual sense of self. This sensation is often referred to within the scientific literature as a sense of presence.
Autonomous voice communication
Autonomous voice communication (also known as auditory verbal hallucinations (AVHs))  is the experience of being able to hear and converse with a disembodied and audible voice of unknown origin which seemingly resides within one's own head.     This voice is often capable of high levels of complex and detailed speech which are typically on par with the intelligence and vocabulary of ones own conversational abilities.
As a whole, the effect itself can be broken down into 5 distinct levels of progressive intensity, each of which are described below:
A sensed presence of the other
Mutually generated internal responses
Internally felt conversational responses to one's own thoughts and feelings which feel as if they are partially generated by one's own thought stream and in equal measure by that of a separate thought stream.
Separately generated internal responses
Internally felt conversational responses to one's own thoughts and feelings which feel as if they are generated by an entirely distinct and separate thought stream that resides within one's head.
Separately generated audible internal responses
Internally heard conversational responses to one's own thoughts and feelings which are perceived as a clearly defined and audible voice within one's head. These can take on a variety of voices, accents, and dialects, but usually sound identical to one's own spoken voice.
Separately generated audible external responses
Externally heard conversational responses to one's own thoughts and feelings which are perceived as a clearly defined and audible voice which sounds as if it is coming from outside one's own head. These can take on a variety of voices, accents, and dialects, but usually sound identical to the person's own spoken voice.
The speaker behind this voice is commonly interpreted by those who it to be the voice of their own subconscious, the psychoactive substance itself, a specific autonomous entity, or even supernatural concepts such as god, spirits, souls, and ancestors.
At higher levels, the conversational style of that which is discussed between both the voice and its host can be described as essentially identical in terms of its coherency and linguistic intelligibility as that of any other everyday interaction between the self and another human being of any age with which one might engage in conversation with. Higher levels may also manifest itself in multiple voices or even an ambiguous collection of voices such as a crowd. 
However, there are some subtle but identifiable differences between this experience and that of normal everyday conversations. These stem from the fact that one's specific set of knowledge, memories and experiences are identical to that of the voice which is being communicated with.   This results in conversations in which both participants often share an identical vocabulary down to the very use of their colloquial slang and subtle mannerisms. As a result of this, no matter how in-depth and detailed the discussion becomes, no entirely new information is ever exchanged between the two communicators. Instead, the discussion focuses primarily on building upon old ideas and discussing new opinions or perspectives regarding the previously established content of one's life.
Autonomous voice communication is often accompanied by other coinciding effects such as delusions, autonomous entities, auditory hallucinations, and psychosis in a manner which can sometimes lead the person into believing the voices' statements unquestionably in a delusional manner. It is most commonly induced under the influence of heavy dosages of hallucinogenic compounds such as psychedelics, dissociatives, and deliriants. However, it may also occur during the offset of prolonged stimulant binges and less consistently under the influence of heavy dosages of cannabinoids.
The experience of communicating with hallucinated voices has been well established with and without the use of hallucinogenic drugs through scientific study. For example, one study successfully demonstrated that anybody can encounter a dialogue between themselves and a voice of unknown origin under the influence of psilocybin mushrooms. This study interviewed 128 participants with an approximate total of 3,427 psilocybin mushroom experiences between them and revealed that 35.9% (46) of the participants reported voices whilst 64% (82) did not. 
Even outside of these drug-induced experiences, hearing voices within one's head is a well-documented psychological phenomena. Generally, it can be considered as a relatively harmless state of mind to find oneself in, in and of itself.  
-  Toh, W. L., Castle, D. J., Thomas, N., Badcock, J. C., & Rossell, S. L. (2016). Auditory verbal hallucinations (AVHs) and related psychotic phenomena in mood disorders: analysis of the 2010 Survey of High Impact Psychosis (SHIP) data. Psychiatry research, 243, 238-245. | https://doi.org/10.1016%2Fj.psychres.2016.06.035
- Romme, M. A. J., Honig, A., Noorthoorn, E. O., & Escher, A. D. M. A. C. (1992). Coping with hearing voices: An emancipatory approach. The British Journal of Psychiatry, 161(1), 99-103. | https://doi.org/10.1192%2Fbjp.161.1.99
- Fénelon, G., Soulas, T., De Langavant, L. C., Trinkler, I., & Bachoud-Lévi, A. C. (2011). Feeling of presence in Parkinson's disease. Journal of Neurology, Neurosurgery & Psychiatry, jnnp-2010. | https://doi.org/10.1136%2Fjnnp.2010.234799
- Hayes, J., & Leudar, I. (2016). Experiences of continued presence: on the practical consequences of ‘hallucinations’ in bereavement. Psychology and Psychotherapy: Theory, Research and Practice, 89(2), 194-210. | https://doi.org/10.1111%2Fpapt.12067
- SherMer, M. (2010). The Sensed-Presence Effect. Scientific American, 302(4), 34. | https://www.scientificamerican.com/article/the-sensed-presence-effect/
-  Looijestijn, J., Diederen, K. M., Goekoop, R., Sommer, I. E., Daalman, K., Kahn, R. S., ... & Blom, J. D. (2013). The auditory dorsal stream plays a crucial role in projecting hallucinated voices into external space. Schizophrenia research, 146(1-3), 314-319. | https://doi.org/10.1016%2Fj.schres.2013.02.004
- Beach, H. J. (1997). Listening for the logos: A study of reports of audible voices at high doses of psilocybin. | http://www.maps.org/news-letters/v07n1/07112bea.html
- Robertson, M., & Toh, W. L. (2017). M109. Phenomenology of Auditory Verbal Hallucinations (AVHs) in a Nonclinical Adult Population and Their Relationship With Mood. Schizophrenia bulletin, 43(suppl_1), S250-S250. | https://doi.org/10.1093%2Fschbul%2Fsbx022.104
- Luhrmann, T. M., Padmavati, R., Tharoor, H., & Osei, A. (2015). Differences in voice-hearing experiences of people with psychosis in the USA, India and Ghana: interview-based study. The British Journal of Psychiatry, 206(1), 41-44. | https://doi.org/10.1192%2Fbjp.bp.113.139048