  We combine Hubble Space Telescope (HST) G102 & G141 NIR grism spectroscopy
with HST/WFC3-UVIS, HST/WFC3-IR and Spitzer/IRAC[3.6\mu m] photometry to
assemble a sample of massive (log(M_star/M_sun) ~ 11) and quenched galaxies at
z~1.5. Our sample of 41 galaxies is the largest with G102+G141 NIR spectroscopy
for quenched sources at these redshifts. In contrast to the local Universe,
z~1.5 quenched galaxies in the high-mass range have a wide range of stellar
population properties. We find their SEDs are well fitted with exponentially
decreasing SFHs, and short star-formation time-scales (\tau<100Myr). Quenched
galaxies also show a wide distribution in ages, between 1-4Gyr. In the
(u-r)_0-versus-mass space quenched galaxies have a large spread in rest-frame
color at a given mass. Most quenched galaxies populate the z~1.5 red-sequence
(RS), but an important fraction of them (32%) have substantially bluer colors.
Although with a large spread, we find that the quenched galaxies ON the RS have
older median ages (3.1Gyr) than the quenched galaxies OFF the RS (1.5Gyr). We
also show that a rejuvenated SED cannot reproduce the observed stacked spectra
of (the bluer) quenched galaxies OFF the RS. We derive the upper limit on the
fraction of massive galaxies ON the RS at z~1.5 to be <43%. We speculate that
the young quenched galaxies OFF the RS are in a transition phase between
vigorous star formation at z>2 and the z~1.5 RS. According to their estimated
ages, the time required for quenched galaxies OFF the RS to join their
counterparts ON the z~1.5 RS is of the order of ~1Gyr.
