A lot of "it depends" depending on the writer, so here are a few scenarios from my past:
What I probably liked best from a writer was charts that had chords, the vocal, and indications of where he wanted specific notes or accents in specific places. The rest was for me to fill in with whatever I thought would be right for the style. Only one song I went "no I can't play that many notes that fast" so we agreed I can just do some basic Latin groove or something. But that guy is a genius jazz pianist.
A couple of singers just did the "I dunno, I put my fingers somewhere, I don't know the note names" thing, one on piano and one on guitar. The piano one was easy enough, everything was simple diatonic triads in spite of not knowing what a diatonic or a triad was, but the guitar one unintentionally did some jazzy extensions in the chords that took some looking at her fingers to figure out what was happening, analyzing what chords it adds up to, and then arranging a bassline for that.
And on the other end of the spectrum, one band I replaced a didgeridoo player, so the preexisting basslines were as easy as it gets - either a C or an F or nothing.
What I probably liked best from a writer was charts that had chords, the vocal, and indications of where he wanted specific notes or accents in specific places. The rest was for me to fill in with whatever I thought would be right for the style. Only one song I went "no I can't play that many notes that fast" so we agreed I can just do some basic Latin groove or something. But that guy is a genius jazz pianist.
A couple of singers just did the "I dunno, I put my fingers somewhere, I don't know the note names" thing, one on piano and one on guitar. The piano one was easy enough, everything was simple diatonic triads in spite of not knowing what a diatonic or a triad was, but the guitar one unintentionally did some jazzy extensions in the chords that took some looking at her fingers to figure out what was happening, analyzing what chords it adds up to, and then arranging a bassline for that.
And on the other end of the spectrum, one band I replaced a didgeridoo player, so the preexisting basslines were as easy as it gets - either a C or an F or nothing.