  Narrow absorption features in nearby supernova (SN) spectra are a powerful
diagnostic of the slow-moving material in the line of sight: they are
extensively used to infer dust extinction from the host galaxies, and they can
also serve in the detection of circumstellar material originating from the SN
progenitor and present in the vicinity of the explosion. Despite their wide
use, very few studies have examined the biases of the methods to characterize
narrow lines, and not many statistical analyses exist. This is the first paper
of a series in which we present a statistical analysis of narrow lines of SN
spectra of various resolutions. We develop a robust automated methodology to
measure the equivalent width (EW) and velocity of narrow absorption lines from
intervening material in the line of sight of SNe, including Na I D , Ca II H&K,
K i and diffuse interstellar bands (DIBs). We carefully study systematic biases
in heterogeneous spectra from the literature by simulating different
signal-to-noise, spectral resolution, slit size and orientation and present the
real capabilities and limitations of using low- and mid-resolution spectra to
study these lines. In particular, we find that the measurement of the
equivalent width of the narrow lines in low-resolution spectra is highly
affected by the evolving broad P-Cygni profiles of the SN ejecta, both for
core-collapse and type Ia SNe, inducing a conspicuous apparent evolution. We
present thus an easy way to detect and exclude those cases to obtain more
robust and reliable measurements. Finally, after considering all possible
effects, we analyse the temporal evolution of the narrow features in a large
sample of nearby SNe to detect any possible variation in their EWs over time.
We find no time evolution of the narrow line features in our large sample for
all SN types
