  As we are entering the era of precision cosmology, it is necessary to count
on accurate cosmological predictions from any proposed model of dark matter. In
this paper we present a novel approach to the cosmological evolution of scalar
fields that eases their analytic and numerical analysis at the background and
at the linear order of perturbations. We apply the method to a scalar field
endowed with a quadratic potential and revisit its properties as dark matter.
Some of the results known in the literature are recovered, and a better
understanding of the physical properties of the model is provided. It is shown
that the Jeans wavenumber defined as $k_J = a \sqrt{mH}$ is directly related to
the suppression of linear perturbations at wavenumbers $k>k_J$. We also discuss
some semi-analytical results that are well satisfied by the full numerical
solutions obtained from an amended version of the CMB code CLASS. Finally we
draw some of the implications that this new treatment of the equations of
motion may have in the prediction for cosmological observables.
