Since we are getting into playing realistic orchestral sounds live... let me say the way to go about it is softsamples. There are too many articulations involved in a real orchestra that is almost impossible to cover using one patch from a synth.
However, having said that, it's not impossible to get near-realistic effects. Firstly, one will need a good patch. And the best (my opinion) is Roland. Next comes the skill. You don't play orchestral patches the same as a piano patch. For one, you don't use sustain pedal as much - you sustain with the fingers holding down. That will prevent creating an unreaslitic mushy sound. Next, you don't play too many block chords. Example - For brass or string patches, one note = one section of instruments (ie 3-4 trumpets and 8-12 strings). Playing a block chord will give you an unreslistic gigantic orchestra. Also, one will have to use the expression pedal very frequently. Then there's the way to actually play it to avoid the dreaded "sucking effect" of the strings, for example. The "sucking effect" is also related to the quality of the patch and it's programming.
To play orchestral instrument live, you have to think like a player in an orchestra, or rather like the conductor. So playing full strings, one will have to think in 4-parts (violin, viola, cello, double bass). Each part is one note, or most 2 for violins (violins 1 and 2). Playing 3 notes in the cello range, for example, will naturally unrealistic to the ears.