Tone: Consistency vs Evolution

I spent the better part of yesterday evening writing my current WiP and eventually typed the final word of the chapter I've been struggling to finish at about five past midnight. This evening I re-read the 2.5k from last night. This led me to realise how the voice and tone of the writing had changed from earlier chapters. I started to wonder if this was because I was being inconsistent or because the tone, along with the character, had evolved.

A previous project, a third-person, world-spanning YA fantasy, told the story with the same voice throughout, the omniscient narrator. My current WiP is a close character narrative, third-person but a singular, inside-the-head point of view that rambles as his thoughts do and follows his mood. When my MC is panicked, the tone changes to short, quick sentences. When he is happy, the sentences are longer, more descriptive, and more jovial.

Here are my top 5 tips for keeping your tone appropriate to your narrative.

1. Decide early on your narrative perspective - 1st, 3rd, close or far and commit to it. There is nothing worse in a novel than reading one chapter of 3rd person, then one of 1st, then swapping throughout - far too confusing.
2. If your story follows one character, keep his/her personality consistent, write with his/her mood in mind.
3. Don't be afraid to let your character naturally evolve, if an event or conflict leaves your previously happy-go-lucky MC jaded and cynical, make sure to change your style to reflect this.
4. Be aware of your characters age, education, and social status, this is also important for dialogue.
5. If your chapters jump from the PoV of one character to another, try to alter your voice so the personalities of your characters come through in your writing.

These may seem obvious but you would be surprised how many stories I've read that fail on these simple points.

How do you write the tone and voice of your story? Do you let it envolve or are you consistent?