The Bee Gees' guitar setup and effects pedals for "Night Fever" are detailed here.

Pedals for Effects:

Especially in the introduction and bridge, you can hear this wah-wah. That strange, almost speech-like noise is produced by it.
To give some rhythmic sections a little more warmth and motion, a slight phaser effect might be applied.
The rhythm guitar has a subtle overdrive for some gritty effect, although it's not quite as extreme as heavy distortion.