A student asked me today how to protect character voice. Her experience with an editor, who slaughtered her character's dialogue and made the character say things not only the author felt were inappropriate and silly, but that went against the character's personality.
In my opinion, she had a bad editor match. A good editor should forage for the source of your disconnect.
Of course, they have to first know there is a disconnect. You have to stand up for your characters. It's not uncommon for authors to have to fight to maintain voice for themselves and their characters.
That' being said, here's something else to think about. The editor may think the character’s voice is weak, and so is beefing it up with changes. So sometimes you have to consider that the writing isn't strong enough and is being changed for a reason.
When an editor gives a suggestion, they are doing so based on the material at hand and how they judge the character from it. If you have not conveyed your character the way you think you have, the editor may judge the character inappropriately. And thus make suggestions not in line with your thinking. This is again an issue of the writer not making the character so strong on the page that the dialogue voice and style are crystal clear (even if the words needs tweaking).
Every once in a while my critique group (3 of whom are pro editors) will give me feedback that isn't in line with my character. I'll tell them that's not who the character is and ask what I've done to create the misinterpretation. Usually this clears up the matter: that somehow in the book I’ve mislead the reader on expectations. Again, it falls to me, the author, to be clearer.
Go forth and be clear.
Your Editor Devil.