  We give a universal formula describing derivation operators on a Hilbert
space for a large class of interpolation methods. It is based on a simple new
technique on ``critical points" where all the derivations attain the maximum.
We deduce from this a version of Kalton uniqueness theorem for such methods, in
particular, for the real method. As an application of our ideas is the
construction of a weak Hilbert space induced by the real $J$-method.
Previously, such space was only known arising from the complex method. To
complete the picture, we show, using a breakthrough of Johnson and Szankowski,
nontrivial derivations whose values on the critical points grow to infinity as
slowly as we wish.
