Next time a bandmate plays an F# or a G# while in the key of C or A minor I'm gonna let it slide because, hey - its in key.
C and A minor may share a key signature, but musically they behave very differently.
So we as bass players have a tendency to obsess, wrongly, over modes. What's really important is the harmony. If you've got a musical situation where the 1 chord is Am, and you have a progression like this:
||: Am | Bm | Cmaj9 | G :||
That feels an awful lot like Am. But one of those chords has an F# in it. That gives it a "lighter" sound, and you could posit that this chord progression is in A dorian. But it's a little bit more clear to simply describe this as A minor and not worry about it so damn much.
How about the "Jam Band Standard?"
||: Am7 | D9 :||
I've seen this described before as a 2-5 in G. But where's the G? I've spend many, many, many hours in bands playing this exact chord progression, unaltered, never resolving. That's because it actually does resolve, because A is the 1 and D9 is the dominant 4. No need for extra mental gymnastics. And yes, A dorian works over this. But so does G major and C lydian and D mixo, etc etc because they are all the same scale.
How about something with traditional functional harmony:
||Am7 | Bm7b5 E7#9 :||
G# becomes an awfully important note in the key of A minor sometimes, because if you want to have a 5 chord, you need a major third above E which doesn't occur in any major scale modes, so we needed to invent the idea of "harmonic minor," which itself works well as a toolbox to rummage through for soloing ideas. You're gonna be playing G#'s a lot in that case.
Now you did mention the key of C. I play F#'s and G#'s all the time as passing tones to nearby chord tones. I'll only use them as extensions, or in the case of G# a #5 chord tone, in certain circumstances. But they're there, available to make interesting sounds. Of course you can't just do this randomly, otherwise you just sound like someone grasping for notes that you can't yet hear in your head, leading to unmusical disjointedness. Generally speaking, minor keys end up with more possibilities for notes to be used in chords and solos because of the V7 chord being so easy to alter and make it fit.
Back to why bass players obsess over modes: we don't have the ability on our instrument to produce meaningful harmony. If you don't have any ability on piano or guitar or another instrument that is designed to play chords, then you're lacking the ability to play for yourself the harmony that you are tasked with supporting as a bassist. That's why I think it's pretty important to know some piano or guitar, regardless of what instrument you focus on as your primary.
*EDIT: In continuing to think about the subject, I think it's important to differentiate the idea of using modes as soloing tools (which in many cases can be a fool's errand) and the idea of modes as their own keys. More on that in the video and paragraph below.*
Highly recommend watching the video below until at least the 10 minute mark. Ted Greene perfectly sums up his view on keys and modes and how he manipulates the sound.
