Depends on the solo. If you take a solo like Seize The Day – that is completely melody driven. The solo on Afterlife would be much more scale/arpeggio based. The key is to make it fit the song. The notes obviously need to fit the key, but the emotion of the solo is also important. What’s the song about? Is is sad song? A love song? A song about war? The solo should reflect that.
I always find it easier to add notes to a solo rather than take notes out. So start with the least amount of notes you can to make it work. Then work on making it interesting. Play the same notes in different ways. Try bending into a note you normally wouldn’t. Slide up or down into a note. That might be enough. If not, you can always add scales, speed, and flash to make it more aggressive or exciting.
My last word of advice is that humans respond and, more importantly, REMEMBER great melodies. They might be impressed with speed – but they don’t remember the notes you played. They only remember you played fast. When they walk away from your show humming a solo you played – that’s magical! And if it’s fast, aggressive song – by all means – shred! But if it’s not absolutely necessary, make your solo about three things.
Melody! Melody! Melody!