If you want to get good at throwing in some "cool sounding chords" on a simple bluegrass tune, there are three concepts that will get you very far in this chord world — all rooted in the approach of Eldon Shamblin, the western swing guitarist who spent many years in Bob Wills' band.
The Three Concepts
1. Voice Leading Applied to both bass lines and chords, voice leading is the practice of moving smoothly between notes and chord voicings with minimal jumps.
2. Inversions Rearranging the notes of a chord so that something other than the root is in the bass — a powerful tool for creating movement and color.
3. The Circle of 5ths A framework for understanding how harmony tends to move. It's not always predictable, but knowing it gives you a map for where chords are likely to go next.
Theory as a Suggestion
If this seems overwhelming, don't worry. These are relatively simple concepts and are mostly just guidelines — suggestions on how to be more creative. At its best, that's all theory is: a suggestion that systematically expands your possibilities.
The Alternative: Just Steal
If you'd rather skip the theory, just listen more and steal. That's a perfectly acceptable alternative.
For your stealing pleasure, check out this video of David Grier. There are lots of walking bass lines and reharmonization going on — well worth studying closely.
https://www.youtube.com/live/z25n-KUGPKM
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