As a student of both the violin and viola, I used to slide my bow back and forth, fascinated by the kaleidoscopic tonal changes that occurred by ever so minute changes in technique:
Pull the bow perpendicularly across the strings so that it nearly touches the bridge, and one produces a wispy effect called sul ponticello. Tap the strings with the wooden portion of the bow, and one is playing col legno. Changes in tempo induce no consequential changes in pitch, but when accented by movements in the wrist or elbow, and depending on speed and strength of the accent, these actions can yield limitless variations in character and tone.
Similarly, tilt the bow along its central axis so that one is either pulling the bow towards oneself, or pushing it away, towards the head of the instrument where the pegs or tuning keys are placed, and you will also introduce a multitude of different tonal effects.
Changes of pressure –how heavy or lightly one rests the bow on a string as it is drawn– is yet another away to affect the character of the sound.
Guitarists intuitively understand the angle of the pick can have an equally significant effect on tone. Similarly, the choice between even using a plectrum, or using one’s fingers to pluck the strings will produce profound effects, not just on the strings, but on one’s life, one’s career, and perhaps one’s relationship to God as well. (Because as we all know, the devil loves rock’n’roll.)
Additionally, the method of contact of one’s left hand upon the violin's neck also produces several appreciable variations. And as with the guitar, the same pitch can be sounded from either a low position on a high string, or from a high position on a low string. The result this affords a player is one of subtle tonal difference between two or more otherwise identical pitches.
The application of vibrato –that is moving minutely on and off pitch– far from making the instrument sound out of tune, also creates an infinite variety of sounds framed by suckle sweetness on one end of the sonic spectrum and frantic anxiety on the other. But why would you do this, besides as an emotive effect? Well, as no two players make contact with their instrument in the same way, nor apply exact speeds of vibrato at the same time, nor can be said to be perfectly in tune with one another at any given moment, a rich choral effect is produced (when two or more string players perform as an ensemble). But strip away vibrato, eliminate dynamics, and though you may have cloned one violin into sixty identical units, what is left is not a powerful symphonic ensemble, but the syrupy singularity we often associate elevator music.
This instant access to variation, in real time, across the breadth of any work, by string players, is one reason why novice arrangers using modern electronic technology often fail to produce realistic string parts, even when working with recorded samples of real instruments. For in order to produce a convincing recreation –and it can be done– each facsimile must somehow be made different than its copies, reflecting minute variations in apparent contact between bow, left hand and strings, note to note, and even within notes.
Using samples from several different libraries is a common approach and aids in adding variety, especially if those samples are then further tweaked by the composer, and their expression modified via the application of high and low pass filters. Of course, it’s cheaper to do this than it is to hire a full orchestra, which is why professional composers will sometimes take the time to deal with such minutiae –or have their assistants do so. And if the budget allows, the samples will eventually be eliminated altogether, or 'massaged' into the mix so that they lay just beneath the surface of an actual orchestra.
It works the other way, too. I can think of many live orchestral dates, which I either produced or supervised, where when working with a small ensemble, we filled out the sound with samples in order to produce a grand orchestral effect on the listener. In this manner, an adept composer can capably convince an audience that a synth track with a soloist is a chamber ensemble and that a 16 piece ensemble is a sixty piece symphony.