Have a brief for a doppler SFX on a voice throughout a 30 second read and than will pass by on a key couple of words at the end of the read.
Whats the best way to approach this?
Off the top of my head, i'd say use a very mild doppler pass at the end, in the key-word pass-by. Doppler plug ins affect pitch, which is fine for cars and whatnot, but the effect of a pitch change on a human voice will stick out like a sore thumb, and potentially sound silly.
If you're after the effect of the voice coming closer and closer, and then whooshing by, you could use eq/panning/reverb/performance to create this effect, and bolster the effect of your slight doppler shift at the end.
Doppler plug ins such as the Waves Doppler give you the option to bypass any combination of elements that make up the effect, or alternatively adjust the 'strength' of those elements. As Roger points out, the pitch element generally has an adverse effect on voices and I would also suggest a very low application of pitch or bypassing it altogether.
I imagine the short length of some words will cause problems and it might be worth adding a reverse reverb tail building into the word, allowing the intelligible part of the word to be at the center of the doppler and then reverbing out again. Depending on your brief this may or may not be useful as it will obviously mess with the natural flow of the read, but on the other hand will assist in highlighting those key words.