This came up on another thread recently, but I'd like to explore it deeper. In that thread I argued that the narrative tone and language must reflect the tone and language of the MC, as they are one and the same, but somewhat hypocritically I find myself neglecting this rule. When I write dialogue I find it very simple to write with a tone and style, but when it comes to narrative it's almost like I put on a 'telephone voice' (I hope we're all familiar with that idiom) and speak in a far more proper and elegant way. One example is that my character would never say the correct, "Oscar and I have already told you..." It would be "Me and Oscar have already told you...", but in the narrative I have to force myself to use the latter because it sounds so off. Is this a rule which shouldn't be broken? Can you have a 1st person narrator who speaks more eloquently that the MC? My switches are never extreme (it's not like my MC has a thick accent of any kind - it's a generic, nondescript semi-northern English accent) so it's just the odd little things, such as the 'Oscar and I' / 'Me and Oscar' conflicts that pop up occasionally. Thoughts?
My first question is whether or not there is a significant age difference in your 1st person MC between the time the story would have occurred and the time in which (s)he is narrating. For example, my 1st person narrator is narrating in adulthood about events that took place in his early teens. So, yes, the manner in which he narrates has echoes of how he would have spoken at the time, but his dialogue is much "younger". But, yes, I think it's important when narrating in first person to get the voice right. You have more latitude when you only have one 1st person narrator. In cases where there are more than one, the distinction between/among the voices is extremely important. My favorite example of an author who didn't get it right is Elizabeth Kostova in The Swan Thieves.
It's an odd one, as in my head we are different people. The MC isn't the same person telling the story. It's a problem I fight to overcome, but it's almost as though I can't let the MC's tone and voice come out through the narrative. The main character is a fictitious person. I need to be him, to inhabit him when I'm telling the story, but I struggle. Does that make sense? Hunter S Thomson pulls it off so well in FaLiLV because that book is practically a biography, so there's no clash of voices. Of course I know there are countless other writers who can do it without this advantage, but I was just using it as an example.
If the narrator isn't the MC, then the MC's voice shouldn't come out in the narration, the narrator's voice should. Think Scout in To Kill a Mockingbird, or Nick Carraway in The Great Gatsby.
I think of first person narration as of the MC is writing his own autobiography, or recording his own story. It makes sense that he doesn't tell his story in his regular "speaking voice". He will make sure that his grammar is correct, and to speak in a slightly higher level vocabulary, while keeping to the same style of speech.
In first POV it is the same character(s) narration as well as dialogue, and therefore should reflect it as such. If your MC is one way at work and another at a pub with friends, the narration should be the same regardless of environment. Obviously the dialogue will reflect this, even if it feels contrary to the narration. The more dynamic the character the more the dialogue reflects the situation, and the narration that is true to the MC. At the very least both narration and dialogue should feel at least a reflection of the persona of the MC at any given moment. Using specific identifiers such as lingo and slang that are apart of the MC's narration style is a big plus (especially when taking on multiple first POVs, but it works well with a singular voice as well).