Applying the chromatic scale

I understand the chromatic scale includes all semitones in an octave, for example: C C# D D# E F F# G G# A A# B

So, where and when should this be used in a bassline? How should they be used tastefully and properly? I understand the concept of passing notes.

Also, since every note is included in a chromatic scale, how does that jibe with the idea of a song being in a "key" where there are notes that belong and do not belong to the key of the song? In other words, in a C-F-G progression, if I played C D D# F-F F F# G there are notes that are not in the key of C, but are in the chromatic scale. Is it correct to use these as passing notes? How do I apply the chromatic scale?