When he emerged in the early part of the century he was considered
"precise" by the standards of the time, but compared to Feuermann,
Fournier, Starker, and most of the cellists that followed them "precise"
is not the first word I'd choose to describe Casals.
You will hear occasional smudges of intonation that become more frequent
as the decades pass, though they never became as pervasive as they did
for Szigeti or Menuhin. I am in no way discounting his musicianship.
IMHO Casals was a supreme musician, musical evangelist, and a giant of a
human being. At the superlative level Casals reached, comparisons
become meaningless. BTW for all of their technical imperfections, I feel
the same way about Szigeti and Menuhin.
DF