Ok here are my 2 cents from a little bit of experience in dialogue editing/mixing:
1) Whistles must be edited out by the editor and fixed before it hits the mixing stage.
I have mixed narration and dialogue with many whistles, and when I was starting out I tried to find the exact frequency of the whistle (2.5 to 4K) to notch out or de-ess with a dynamic notch EQ or Oxford Suppressor, what have you, and the amount of time I spent trying to fiddle with de-essers and parameters to make it breathe well with the rest of the voice, it would have saved me 80% of the time if I just snipped out the whistle. I can see them now in waveforms and I have a CH and S sound copied at the end or beginning of the session to fly in if I come across a bad whistle. It sounds so much better and doesn't give you unnecessary processing on the rest of the voice if all you're trying to handle is 2 or 3 whistles in one minute of dialogue or narration.
2) EQing a voice is different for every voice. I have heard mixers tell me "I ALWAYS add 'air' to the voice around 16K and notch out the 2.5K automatically" etc., and this is utter rubbish. Every recording is different, different acoustics, different mics, different recording equipment being used. Each voice you mix will require you to first hear it newly and EQ it specifically for that voice. I have never come across 2 different recordings having the exact same EQ. You have to listen.
3) Absolutely run low-pass filters on a voice if his back is to the camera. Your reference is any Pixar film (specifically The Incredibles) when Samuel L. J's character is searching his house for his super-suit, and his back is to the camera, and his voice is muffled, and he turns to the camera to say "Woman! Where is mah supah suit!!!" his voice is all of a sudden more present and brighter - it adds so much more dimension to the mix when the voice is mixed like that. It's a volume-ride and low-pass filter + reverb ride type of thing.
4) Something I learned from music mixing but also applies to final mixing, which is you EQ the voice with everything in the soundtrack playing - don't EQ it solo. You can set a temp EQ with it in solo but always check how it sits in the mix with all the other elements.
5) Adding early reflections to a voice can add a lot of realism and presence to a voice that's not possible with EQ.