The bright Seyfert 1 galaxy MCG-6-30-15 has provided some of the best evidence to date for the existence of supermassive black holes in active galactic nuclei. Observations with ASCA revealed an X-ray iron line profile shaped by strong Doppler and gravitational effects. In this paper the shape of the iron line, its variability characteristics and the robustness of this spectral interpretation are examined using the long XMM-Newton observation taken in 2001. A variety of spectral models, both including and excluding the effects of strong gravity, are compared to the data in a uniform fashion. The results strongly favour models in which the spectrum is shaped by emission from a relativistic accretion disc. It is far more difficult to explain the 3-10 keV spectrum using models dominated by absorption (either by warm or partially covering cold matter), emission line blends, curved continua or additional continuum components. These provide a substantially worse fit to the data and fail to explain other observations (such as the simultaneous BeppoSAX spectrum). This reaffirms the veracity of the relativistic `disc line interpretation. The short term variability in the shape of the energy spectrum is investigated and explained in terms of a two-component emission model. Using a combination of spectral variability analyses the spectrum is successfully decomposed into a variable power-law component (PLC) and a reflection dominated component (RDC). The former is highly variable while the latter is approximately constant throughout the observation, leading to the well-known spectral variability patterns. (Abridged)
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