I was working on some tracks today - just dipping in and out of some unfinished material. I had my fender telecaster plugged in to my focusrite preamp and into my mac. The track I had open on GarageBand was "TP GROOVE" (TP stands for Tom Petty) - it's only a temporary title until I know what the song is gonna be about. I played the track through and started noodling around on the guitar, solo'ing bits and adding fills here and there. It sounded totally shit!
So I changed the amp setting and suddenly realised that the laid back groove of the track was more suited to a Mark Knopfler style. I selected a setting called "smooth soul" and it had a clean, round, jazzy snap to it. I threw the plectrum down and played the strings with my fingers - a la Dire Straits and suddenly it fit the track! Wooohoo!
But it needed something else? I continued to play along with the track, recording as I went and just letting the notes flow. Whatever I played, good or bad, it was going down on the track to review later. But what was it about the sound that wasn't quite right? AHA!!! It needs some echo.
Added an echo pedal in the edit window and played around with the settings. I didn't want each note to repeat TOO much because it just sounded dirty and confusing. I eventually settled on a setting where each note was repeated but only for a few milliseconds. It made the sound more interesting and actually made my playing sound better too! WHICH WAS GREAT!
I carried on for about an hour, recording various bits of fill and stuff and then pressed "save". I would return to it another day, after I'd let the ideas and the "sound" mature in my head for a bit. Of course, until I have the lyrics sorted out, I won't know where the fills will go - but at least I know what SOUND it will be.
So I find that letting it flow and seeing what notes your fingers produce from the strings is the ideal way to come up with a great solo or fill. It will come eventually. You just gotta have patience!