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?
Well, I would say that for many if not most purposes, "applying the chromatic scale" is maybe the wrong way to look at it. You generally wouldn't apply it in the same way as you would apply, say, the dorian mode, because as you note, there's no note selection involved--all tones are permitted. Therefore, the chromatic scale doesn't relate to or serve to establish any key or modality, which is what you're usually thinking about when you think about "applying a scale." (There is music that's strictly chromatic, like twelve-tone music, but that's another kettle of fish and you probably don't want to go there.) For most of us in most situations, I'd say it will often turn out to be more useful to ask instead, when can I effectively use chromatic nonchordal tones within the harmonic structure I'm in? Like, for example, when might I use an F# if I'm ostensibly playing in C major, or when might I use an Ab if I'm mainly in D dorian? Again, it is possible to play chromatically, but in practice, if you're playing to chord changes and trying to sound somewhat consonant, even if you use all the notes in the chromatic scale, probably what you're really doing is not so much playing purely chromatically as it is playing within some key or mode and using chromatic approach-passing-departure notes for color.