OK. I,m gonna take a shot at this. First, there are technique differences. Classical vionlinists use more different techniques such as vibrato, bowings with weird names--spizzacato, con legno,left hand configuration, etc. Then there are posture differences. Yes, it is usually true that most violinists are not loose enough to play fiddle style. Most violinists stand more still than fiddlers; they often appear to be robots with a difficult bowel movement. I spent at least 10 yrs. learning how to sound like a fiddler, instead of a violinist playing fiddle tunes.
I think it is true that some classical training makes a better fiddle player.....Allison Krause, etc. There are of course all kinds of fiddle players, from Allison, et. al. down to the 90 yr. old man down the block who owns a Sears Roebuck fiddle, never took a lesson and taught himself everything he knows, (with the help of a banjo player so he can cross tune) Violinists seldom cross tune. The stare AGHAST!! Fiddlers with smoe classical training have a jump on those without, because they can do several different kinds of vibrato when it is called for. They usually have their left hand in the proper position for left hand pizzacato, can use long whole bow strokes for waltzes, and short choppy ones for jigs. Fiddle players seldom play above 1st position..a few tunes call for 2nd or 3rd, but not many. Classically trained players have no problem with this, and can use different fingerings and positions to make the tone color match. (That is why classicists use 4th finger so much...the tone color of a fingered note does not equal the tone color of an open string, even if they are the same pitch).
Yes, violinists have trouble with improv.which is the backbone of such styles as jazz, blues, and Celtic among others. They are trained that if it is not on the paper you don't play it. Beethoven does not take kindly to people adding turns and grace notes just because they think they should be there. Whick brings us to ATTITUDE differences........if a fiddler wants to sneak something in, he/she does. No one cares if you spill beer on a fiddle. If a violin needs repair, a violinist buys a ticket on an airplane, buys his violin a ticket, and flies halfway round the world to get a new bridge. A fiddler whips out the old pocket knife and trims a new one. If a violinist lost a left hand finger joint in a table saw, he would quit playing. The old man down the street just compensates by using 3rd finger for his 2nd. It's a tough act to follow.
Then, there is set up--violins have a high arched bridge to facilitate playing only one string at a time. If they want double stops, they have to go close to the fingerboard. Violinists usually use gut or Perlon cored strings bacause they are mellower and don't ring. Fiddlers want double stops, so the bridge is flatter, and they may need to walk on 3 accordians, or play in a huge outdoor setting, so they use steel strings. Also, on a fiddle the nut is sometimes lower which helps stop left hand contortion-ism when playing chords.
AAARRRGGGHHH! didn't mean to go on so long...sorry! :>) Sorcha