I once saw a TV programme with Yehudi Menuhin playing with "The 40 Shetland Fiddlers". When they played together the effect was very trad/Shetlandy but when Yehudi played a solo it was very classical i.e. I found there was a certain rigidity about his playing of a jig (or whatever). I think there is a certain solemnity about playing the 'violin' which is much less obvious in fiddle playing, even in slow airs.

About 35 years ago I played in a folk group with a very good trad whistle player and a classically trained violinist (I think he was from the SNO) and we had the same problem. It was always "lighten up Norrie!"

Another problem with him was he could not adjust to playing by ear! Can you imagine how hard it was in about 1967 to get sheet music for "The Wild Rover" etc?