A lot of guitar players who are good (or at least, adequate) accompanists have spent a lot of time practicing and developing their chops. I’ll bet that you are pretty comfortable playing 8th note triplets or 16th notes at a fairly brisk tempo. The drawback to developing that level of comfort playing up tempo passages is that it becomes easy to lose sight of how many notes you’re playing. An eighth note run over a measure and a half falls so comfortably under your fingers that it seems like nothing fancy. However, in a moderate tempo song with a leisurely paced melody that eighth note lick can seem awfully busy and may compete with rather than complement the vocal.
It’s a good idea to practice moderate tempo material as well as up tempo tunes. If you have some songs or recorded material or a friend that you play along with, here’s something to try. Take a moderate tempo (90 – 120 beats per minute) piece that you know fairly well. Play fills and licks in all of the places you normally would but consciously use only one note per measure. It doesn’t have to take the whole measure, but only use one note per. When you are only playing one note you have to choose it much more carefully than you would without that restriction. The other choices you make in this exercise are when to play it and how long to hold it. The note could be played on the second beat and held through the measure or played on the upbeat between beats 2 and 3 and held for 1 beat. There are a lot of possibilities with that single note. Your creativity can be really challenged.
After you have practiced that for a while do the same thing with two notes per measure. Do you play equally spaced notes or at the end of the first measure and the beginning of the next? Continue on with three and then four notes per measure and then try combinations. None of this should challenge your technical skills but it will provide a good workout for your creative and emotive abilities and increase your awareness of how much you are playing. Down the road, when you choose to insert the speedy little triplet run it will really stand out in contrast.