The process of improvisation is a peculiar one. You may think you have a grip on it already, yet it keeps eluding you. Let’s just point out one thing right away. Scales and intervals and chords and all that theory stuff are NOT the improvisation itself. They are the devices one can use and integrate into his own style. Just like listening to music helps memorizing all these things theory describes, even unconsciously. There are a lot of people out there who learn all the music theory in the world but they can’t play a single short improvised line of their own to save their lives.
Listen and enjoy what you listen
I think listening to music is the very first, natural step in this whole improvisation game. It starts as early as being a little kid, or even a baby. Of course listening really only works if you like or at least appreciate what you’re listening to. Otherwise, you won’t pay attention in a focused way. Then after listening to and learning a lot of music, a person with good ears develops a song vocabulary, but not only that; he/she will have all the little nuances and details of all those tunes memorized, including the intro, the groove, riffs, solos, etc. so when he’s playing all those songs in his mind or whistling, humming them in the bathroom or wherever he is, he will be able to recreate those riffs and things without having to hear the original at the same time.
The playful steps of creation
Then comes the next step. With the help of all those riffs and solos and little musical things, he will be able to modify some parts of the now well known songs he likes or even change them entirely. It’s composition on the fly. It inevitably happens after some time, using the above mentioned building blocks, during those inside-the-mind listening courses or bathroom whistling “sessions”. It comes naturally for some people. It’s like playing a game that never gets old. And it all happens in the mind first. That’s what I believe is the most important thing, one that theorists tend to neglect. You have to be able to create your own ideas. And you will be able to do it, naturally. You will be singing or whistling them little melodies and harmonies. Or you’ll be hearing them in your head. The particular occurrence doesn’t matter as much as the fact that they have to exist as a thought of yours first. Putting it on the instrument – be it the guitar or a sax or your own voice – that’s just a matter of time and patience to make it happen.