Quest For The Real ,
To make synthesized instruments have that real feel and sound is an ever enduring endeavor for many that is if that is the objective , sometimes of course the goal is to make what is real sound synthetic for example ; a vocoder used on a real vocal or a guitar driving a string synth , yah sometimes you want those weird robotic or clippie or strange OTHER WORLD sounds and distortions but good distortion and bad distortion is another topic and a tangent so for now at least that will not be a consideration .

The first thing you need to do is vary the velocity in such a way as to emulate a human player .
So what does this mean exactly , simply randomizing the timing and velocities will not cut the mustard I mean it is good but still far away from the goal .
People do not play instruments in a random manor that is completely insane and that is what most VST’s and standalone attempt to use and commercialize as though they have the real stuff down pat and it is just simply not true they are just selling you a lie.

They want to piss in your ear and take your money and then laugh about it.

The 2nd thing you need to do is to emulate natural characteristics that is to say make them reactive .
For example, a microphone does not have a truly flat EQ , no none of the properties and attributes of a microphone are flat like a pancake nor ought any of the parameters used to attempt to emulate them be ,

Same for instruments a guitar for example ; each and every note do not have the same exact reaction to the same force applied even the same notes of different strings differ to the same exact applying force even if applied exactly in the same way ,

I am not talking massive variance here it is slight but nonetheless when you hit a A note on another string it varies reactive properties from the other notes even other ‘ A ‘ notes.

Basically reactive is curved and not a strait curve at that , I guess in a sense that could be called a distortion but it is the natural difference between the synthesized and the real. The reactive curves of variance emulation instruments can also be seen as a signature of that instrument or microphone.

Then after obtaining all this variation then a very slight random range might be added to the reactive curves of independent notes just to make things a bit more interesting.