My main character is the host body of another entity, how should I show the entities voice inside the characters head? Would I need speech marks or just maybe in italics?
I apologise in advance for inappropriateness but my own devious character, that which hosts the Seths and the Lokis of this world, quite unfortunately sees your moniker as the host of another entity. Sorry about this but I can't get past the first five characters of your name without reading the rest and having a little snigger. Did you do this on purpose? For the entity's voice: I don't think there's a hard and fast rule, I've seen a thread of inordinate length right here in the forum https://www.writingforums.org/threads/italics-for-thoughts.32989/ on such a subject that I'm sure also reaches a vague conclusion. If I was to do this, I would opt for the italics, that's only my subjective view mind, but I think this would then leave scope for you to keep the real world speech marked by its usual... well marks. The key thing I'd say would be some rigid consistency and by that the delineation would be obvious.
I did not do this on purpose; however when I first noticed it I did snigger myself. I am quite fond of the username so that will not be changing anytime soon! I suggest sniggering at it as much as you like. I am not bothered by the fact that my name includes the word "anal", do you also snigger at the word analyse?
In my WIP, one of the MCs has a, well... separate personality, I guess. It's a long story. Anyway, when she and her alter are talking to each other, I just use normal speech tags - it seems to personalize the other entity more. Italics, on the other hand, make it feel more alien. So, really, it depends on what the entity is, and how you want the reader to view it.
It depends on how you want to portray the sensation of the voice. I've seen voices inside a character' head done "like this" (the character hears it like a real voice), like this (it seems just like a regular thought), and <like this> (it penetrates into the character's brain like a radio signal.)
I want the reader to see it like, if you've ever seen the cartoon Yu-Gi-Oh, like how Yami Yugi talks to Yugi. So it's like how you would talk to yourself except it's a separate entity that exist within the hosts body. It's not really a radio signal, however its supposed to be fluid like a thought but at the same time, it's not a thought. It's very difficult to describe. Kinda like, if you had a separate entity in your mind, but you could suppress it yet still hear it talk to you in a conversation with yourself sense?