Dependent on a few factors. How long the run needs to be - half a bar, a bar, a couple of bars. What key it's in - as in what sort of scale notes are already used. It could consist of pentatonic notes, major or minor notes, or chromatics. Could be, and most likely will be, a mixture.
For faster playing. legato works well, where notes are hammered on and pulled off. Some prefer to play each and every note picked, in which case a decision has to be made as to whether it's alternate picking, hybrid picking, or a mixture.
As far as actual notes are concerned, without a chord sequence and a specific gap to fill it's not easy to say 'do this' 'do that'. You need to look at the last note played before the 'fill', and the next one needed that's the target. Then work out how long, in beats the gap is. Decide if you want really fast notes, or slower ones - the faster ones will, surprise, need to be more in number! Count your way through, find out how many - slowly - then see what's available. Because it's going to be a bit of a flurry, 'out of tune/key' notes will still work, although when played slowly will sound odd. Find a line which fits - it may go in one direction, or change a couple of times, before the target is played - louder than the flurry. Work from there.