When a writer sets out to tell a story, she has a lot of choices to make, and every time she makes one, it influences what options are still available for the other choices. In some cases, one decision can completely eliminate all other options.
Take the matter of narrative voice (which I define as the way all the stuff that isn’t dialog sounds). Theoretically, the writer has three basic options: 1) the narrative can be in her, the author’s, natural voice; 2) the narrative can be in the voice of the viewpoint character; 3) the narrative can be in the voice of an independent narrator who is not the author – essentially, in the voice of an imaginary character who isn’t the viewpoint character (an example of this would be the Paarfi novels by Steven Brust).
If, however, the writer chooses to use a first-person viewpoint, options 1 and 3 disappear. All of the various styles of first-person viewpoint, whether they’re memoir, diary, letters, stream-of-consciousness, etc., are told by the “I” character, so all of the narrative has to sound like that character. Word choice and grammar has to be the same – if the first-person narrator knows nothing about trees, he’ll say “I sat under a tree;” if he knows quite a bit, he might say “I sat under a white oak;” if he’s a botanist, he’ll say “I sat under a specimin of quercus alba.“ This is relatively obvious when the first-person narrator has a strong voice, especially if the character uses unique syntax, word choices, or grammar. It gets less obvious (and harder to do) the more the narrator sounds like the writer.
Where this really gets tricky is in the matter of dialog. A first-person narrator will sound more or less the same in both their dialog and in their narrative (though most people do have a slightly more formal writing style than they do speaking style, so there’s some variation). The dialog that is spoken by other character – the non-viewpoint characters – has to sound like them, rather than like the narrator, yet the first-person narrator’s voice may creep into the way he reports the dialog (unless he happens to be the sort of person who is obsessive about reporting exactly what everyone else says, rather than telling it all the way he remembers it).
In third-person, the writer can choose any of the three options for voice. If the narrative is filtered through the eyes and mind of a tightly-focused third-person viewpoint character, the effect will be almost the same as if the author were using a first-person viewpoint; if the narrative is in the voice of an imaginary character or in the author’s voice, the dialog and the narrative will contrast. How much they contrast is obviously a function of how different the voices of the characters are from the voice of the author or independent narrator.
Manipulating the narrative voice is usually a lot more subtle than, say, changing the tense or the viewpoint, but once one is aware of it, playing around with voice can be fun.