If anyone here is familiar with Heifetz' arrangement of Prokofievs Masks from Romeo and Juliet, could you please be kind enough to give me some tips for the painfully difficult (at least they are for me) fingered octaves? Thanks!
Not familiar with the passage, but some general pointers on the technique:
Fingered octaves are simply something you have to do. Yes, they seem impossible in the beginning (as did every other aspect of playing the violin) and may take weeks to get some comfort with. It requires an extended period of daily practice, and not too much at a time lest you risk injury. If there's a magic bullet, I don't know about it. It's like jogging for non-joggers: take it easy in the beginning and stick with it.
Camilla the only thing to know (but you probably know that already, and Frank wrote it already as well) is to place the higher finger first, only then the lower finger, or at least use a hand position that would obtain if you did it in that order. Also ease in the wrist a little bit (my hobbyhorse). For the rest like Scott wrote, it is just slow practice, gaining flexibility.
Also, know that there is not an effective, long-term way to *tension* yourself into playing fingered octaves. The end effect of the hand's shape must come from looseness and flexibility as others have mentioned.
Do you usually have issues playing fingered octaves, or just these particular ones?
Fingered octaves in scales going up are fun...coming down, not fun!
I'm currently (re)learning fingered octaves because I'm working on the Bruch.