Let's start with the obvious things first. It truly doesn't get much simpler than this. This makes those spaces particularly attractive spots to throw in a fill. Again, there's nothing phenomenally complicated here. Fill B utilizes a slide from the A note to the B and then some pick-offs to get us back again.
Mood is important, too.
If I think it s important to have something to steady the beat, then I will not only play something simple, but play the same one over and over again. If I m being a bit playful, then who knows what fill might pop up at its designated place. Any riff or fill you learn is meant to be played with, to be tinkered with so that you can use it when and where you think it might add a bit of zest to a song. Add an additional note or two here or there. Stretch or shrink the timing to your liking.