It seems like no matter how closely I place one finger next to another a half step always comes out sharp - and I don't have fat fingers. I can never seem to play in tune in high positions because of this. Are there any secrets to playing in tune in high positions? How do the pros do it?
Depending on your scale and the musical passage the half step may be played with the same finger. Playing A Major scale, as an example, has some forms where you shift to the 5th position on D and then play both the G# and A with the 4th finger.
In other cases the previous finger is lifted back and away on the transition when placing the next half step so as to give the next finger some room.
Itzhak Perlman is the classic example of a fat-fingered violin virtuoso. The way he describes what is required is that the new finger pushes the old finger out of the way. The problem with sliding the same finger up is that you get a slide. If the notes are supposed to be slurred you can't really lift the finger up and put it down again half a finger-width up.