[en] Aims. We discuss and characterize micro-lensing among the 3 brightest lensed images (A-B-C) of the gravitational lens system RXS J1131-1231 (a quadruply imaged AGN) by means of long slit optical and NIR spectroscopy. Qualitative constraints on the size of different emission regions are derived. We also perform a spectroscopic study of two field galaxies located within 1.6 arcmin radius from the lens. Methods. We decompose the spectra into their individual emission components using a multi-component fitting approach. A complementary decomposition of the spectra enables us to isolate the macro-lensed fraction of the spectra independently of any spectral modelling. Results. 1. The data support micro-lensing de-amplification of images A and C. Not only is the continuum emission microlensed in those images but also a fraction of the Broad Line emitting Region (BLR). 2. Micro-lensing of a very broad component of Mg II emission line suggests that the corresponding emission occurs in a region more compact than the other components of the emission line. 3. We find evidence that a large fraction of the Fe II emission arises in the outer parts of the BLR. We also find a very compact emitting region in the ranges 3080-3540 angstrom and 4630-4800 angstrom that is likely associated with Fe II. 4. The [O III] narrow emission line regions are partly spatially resolved. This enables us to put a lower limit of similar to 110h(-1) pc on their intrinsic size. 5. Analysis of Mg II absorption found in the spectra indicates that the absorbing medium is intrinsic to the quasar, has a covering factor of 20%, and is constituted of small clouds homogeneously distributed in front of the continuum and BLRs. 6. Two neighbour galaxies are detected at redshifts z = 0.10 and z = 0.289. These galaxies are possible members of galaxy groups reported at those redshifts.