Do you have any of your guidelines that you follow?
The challenge is you want the fills to be appropriate for the context.
Just because you hear a space does not mean that space requires filling.
It's about listening, feeling, and consciously responding o the music
It's a bit hippy - dippy in a way
my approach is basically parts:
1.) master the simplest possible line that works
2.) listen to the music
3.) put your feel of the music into the simplest possible line that works
4.) listen to the music for appropriate places
5.) play appropriate fills
1.) master the simplest possible line that works
know it so well it can be executed by instinct, using minimal sufficient focus to execute well.
know it so well that it runs inside your head at all times, as a reference to which you can return.
2.) listen to the song
after you have mastered the song you can actually start to listen.
grow your intuition of what every instrument is doing
this may expose places you can simplify more - and get out of their way.
hippy dippy part :focus awareness to the emotion of the song.
"strecth out with your feelings, Luke"
3.) put your feel of the music into the simplest possible line that works
this is the part that can trip players up: play technically the same line
but with a feel and intensity that follows the song.
A colleague's college instructor once explained:
"it impossible to increase musical intensity without increasing speed or volume"
free of focusing on execution, you can focus on feel
4.) listen to the music for appropriate places
I find that once I get a handle on 3.) , appropriate places for fills stick out like a beacon.
It's places where others need no space, where the feel of the song does not require full focus on intensity of the core bass line
5.) play appropriate fills
at this point I generally hear the fill in my head before I try to play it.
usually it's either some lead up to the next part or an emotional punctuation of the preceding section.
Sorry for the lack of actual music theory
but this is my best assessment of how I approach it.