According to the Wikipedia article on Spanish phonology, the phonemes /b/, /d/, and /g/ are realized as approximants or fricatives instead of plosives in all but certain contexts (after a pause, nasal consonant, or a lateral consonant followed by /d/). Where or when are they realized as approximants, and when as fricatives?
If you look at the reference on Wikipedia, you'll see that it's not a matter of sometimes they're fricatives and sometimes they're approximants, but rather it's that scholars seem to be in disagreement as to whether they are fricatives or approximants (altogether). In the past, they were called fricatives, but new analyses seem to indicate that they are actually approximants because of the lack of turbulence.
In any case, the spirantized versions [β, ð, ɣ] are realized intervocalically.