  Ionized carbon is the main gas-phase reservoir of carbon in the neutral
diffuse interstellar medium and its 158 micron fine structure transition [CII]
is the most important cooling line of the diffuse interstellar medium (ISM). We
combine [CII] absorption and emission spectroscopy to gain an improved
understanding of physical conditions in the different phases of the ISM. We
present high resolution [CII] spectra obtained with the Herschel/HIFI
instrument towards bright dust continuum sources regions in the Galactic plane,
probing simultaneously the diffuse gas along the line of sight and the
background high-mass star forming regions. These data are complemented by
observations of the 492 and 809 GHz fine structure lines of atomic carbon and
by medium spectral resolution spectral maps of the fine structure lines of
atomic oxygen at 63 and 145 microns with Herschel/PACS. We show that the
presence of foreground absorption may completely cancel the emission from the
background source in medium spectral resolution data and that high spectral
resolution spectra are needed to interpret the [CII] and [OI] emission and the
[CII]/FIR ratio. This phenomenon may explain part of the [CII]/FIR deficit seen
in external luminous infrared galaxies. The C+ and C excitation in the diffuse
gas is consistent with a median pressure of 5900 Kcm-3 for a mean TK ~100 K.
The knowledge of the gas density allows us to determine the filling factor of
the absorbing gas along the selected lines of sight: the median value is 2.4 %,
in good agreement with the CNM properties. The mean excitation temperature is
used to derive the average cooling due to C+ in the Galactic plane : 9.5 x
10^{-26} erg/s/H. Along the observed lines of sight, the gas phase carbon
abundance does not exhibit a strong gradient as a function of Galacto-centric
radius and has a weighted average of C/H = 1.5 +/- 0.4 x 10^{-4}.
