Beyond the basic chords (say C, F, and G or G7 in the key of C), it's possible to substitute chords. Rather than sitting for several measures on C before you change to an F, you may want to go to Am for one or two measures, then F. If it sounds okay and you're used to doing that way, if you find yourself playing along with a bunch of other people who just sit on the C, the worst you can do by going to an Am before the change to F is to convert the C to a C6 for a measure or two. Hardly catastrophic, and it certainly wouldn't screw up the general harmony (no horrible dissonances). Beyond the basic essential chord changes (to keep the chords from clanking with the melody), these "substitute chords" that you can stick in for harmonic variety are pretty arbitrary.

Don Firth