  We present a comprehensive catalog of high signal-to-noise spectra obtained
with the DEIMOS spectrograph on the Keck II telescope for a sample of
F850LP<22.43 (AB) field spheroidal (E+S0s; 163) and bulge dominated disk (61)
galaxies in the redshift range 0.2<z<1.2. We examine the zero point, tilt and
scatter of the Fundamental Plane (FP) as a function of redshift and
morphological properties, carefully accounting for luminosity-dependent biases
via Montecarlo simulations. The evolution of the overall FP can be represented
by a mean change in effective mass-to-light ratio given by <d \log (M/L_{\rm
B})/dz>=-0.72^{+0.07}_{-0.05}\pm0.04. However, this evolution depends
significantly on the dynamical mass, being slower for larger masses as reported
in a previous letter. In addition, we separately show the intrinsic scatter of
the FP increases with redshift as d(rms(M/L_{\rm B}))/dz=0.040\pm0.015.
Although these trends are consistent with single burst populations which formed
at $z_f>2$ for high mass spheroidals and z_{f}~1.2 for lower mass systems, a
more realistic picture is that most of the stellar mass formed in all systems
at z>2 with subsequent activity continuing to lower redshifts (z<1.2). The
fraction of stellar mass formed at recent times depend strongly on galactic
mass, ranging from <1% for masses above 10^{11.5} M_{\odot} to 20-40% below
10^{11} M_{\odot}. Independent support for recent activity is provided by
spectroscopic ([\ion{O}{2}] emission, H\delta) and photometric (blue cores and
broad-band colors) diagnostics. Via the analysis of a large sample with many
independent diagnostics, we are able to reconcile previously disparate
interpretations of the assembly history of field spheroidals. [Abridged]
