We get a lot of business and not artistic oriented posts here so I like to occasionally make / pass along some artistic suggestions. I know many people here score soley electronically with sample sets. And so technically you can do anything you want with instruments. Nonetheless, you may still want to read on since these are not only suggestions for live performance but they will also make your instruments sound more 'real'.
Rule 1: Write idiomatically.
When I first started, coming from a piano background, I thought of all my instruments in the 'piano paradigm. Most of us pianists and guitarists think like that. So when you score other instruments, you tend to end up with a instrument sound that sounds like pianos and guitars and not like trumpets and violins, for example. Now that sort of sound is still quite legitimate and often especially on short timetable projects like TV and web series, you just have time to play instrumentation on a keyboard.
But to take maximum advantage of the instrument you are are scoring with, you want to take advantage of the different ways the instrument can produce it's 'sound'. There are lots of ways to bow a violin or even pluck it, for example and a trumpet or trombone can be play soft or hard or with a mute, for example. One of the best experiences I had when I first started out was I was told I needed to take 1 1/2 years of violin lessons before I wrote for string quartets. I had a similar experience for woodwinds (1 1/2 years of clarinet). It is important not only to understand what the instrument you are using is capable of but also how the musician actually plays it.
The instruments to be realistic should not be used just for 'stacking' notes in a chord like a piano or guitar. It needs to play some sort of melody - main melody, counter melody, inner melody, etc. And it need not play all the time. Give the instrument some 'space'. One of the things I do when I score many staffs, is to play each one as a solo to hear how it sounds. My mentor/teacher reminds me that scoring is not about 'vertical composing' but 'horizontal composing' - everything should stand by itself, fit within the group it is in (strings, woodwinds, saxes, etc.) and make sense within the whole score.
With the idea of idomatic horizontal writing, we come to two more "rules":
Rule 2: Breathe
Another way of saying that is 'less is more'. What I mean is that we certainly have the ability of writing a string of 50 notes in a row for an instrument but in real life, musicians don't do that for two reasons: (1) it may not be physically possible. When writing say for a trumpet (brass instrument) or a sax or flute (a woodwind instrument), the notes are played using breath and air. Musicians have to take breathes to play their instruments. So when you do horizontal writing - especially at relatively fast tempos (I'm currently scoring at 230 beats/minute), you need to put in 'rests'. Sometimes whole bars at fast tempos or at least two quarter notes. Even string players do occasionally stop moving their bows, say to move the bow in the other direction. And (2) often said for Jazz improvisation but valid of pretty much all genres is that silence is part of the melody. Often time, by stating a series of notes in slightly different ways (which we often refer to as a 'motif'), we must give the listeners a chance to absorb what is being musically stated and so the lack of notes to separate the musical thought is often quite useful and perhaps even necessary.
So just because we may have many staffs in our track, does not mean they have to all be active at the same time. Sometimes a single instrument solo with notes and spaces may be all that is needed. A beginning composer may feel that the lack of rhythm or chords with backing instruments may be necessary when in fact (and I can't remember how many times I have been told this myself) - the listeners are very good at hearing what is not being played - both rhythms and chords.
Rule 3: Don't Get Married To Your Notes
I almost always when I think I am done, listen to every instrument one at a time by itself to determine if (a) it has something to say and contribute by itself as a horizontal layout of notes and more importantly (b) does every note make a contribution or saying it in another way - can I remove one or more notes and still state (and probably hear) the same thing. So for example, how many instruments are doubling a particular note would be a decision to get rid of a note 'vertically' when deciding how the instrument participates. But more importantly 'vertically' I would ask things like for example, "does every note which comprises a G minor 7th chord need to be played by the instrument? Can I remove the last note or change the quarter note Bb to a half note and remove the next D? Must there be a 'passing tone' to get that instrument to the next bar? Is the instrument playing a note which is doubling the melody note if it is not part of the melody (sometimes it could be).
When I was first starting off, I wrote notes for everything and got emotionally tied to them. If I had a reason to put them there initially then they must belong. They were 'my notes' and so they could never be removed. What I now find is that I can often go through a single page of score (say 32 bars, for example) and find ways to remove 10, possibly more notes and the score still sounds great. In fact, by removing notes that really don't contribute, I have thinned the score and that allows the lead instruments along with the supporting instruments to be better heard. And that holds true whether it is a fast and loud brassy superhero style them with lots of percussion or a slow, light and thinly string dominated theme. So I also try to remember that getting rid of the notes can actually be used both to 'accentuate' a melody by getting rid of what's in the way AND it can create 'space'.