When given an original composition, I typically ask the composer to tell me what they are feeling. From there, I literally "hear" bass lines in my head.
Wish I could make enough $$ doing it for a living. Even when we did cover tunes, my mates would often say, man your bassline is way better than the original. We were, however covering 80's rock tunes, so wasnt really trampling on any great bass lines.
Point is, I couldn't even help myself with covers - like sit back cop the lines and drone it. Not me. Particularly for the very "plain and white" songs where I tried to interject a wee-bit-o-color.
Mellencamps "Pink Hosues" done in slap, for example. And other tunes are so, uh, "rigid", there wasnt a darn thing I could do with them. I still have a mental block on this tune called 'Jenny" or 8675309 or something like that. You cant put any soul in that "stuff", its so nasty and 4/4'y.
Then again, a 51 yr old bald white guy cant really make any money covering metal, stoner doom, acid funk either. Or I could really tick off the local union bassists who do it for a living ( and have great chops!) by telling the few local soul/RB/funk bands with a horn section that I'd do it all for free. Just for the experience!
Trapped in creative anonimity and therefore poverty........ and I don't mind. Heck man, not to get too poltiical, but I live in a society that seems completly whored out. From politics to religion. All whored out. And I mean no disrespect for honest working girls. Maybe more trustworthy than any eRected representative. Nor do I mean that cover band junkies are "whoring out". No.
Only that I am fortunate to make a living otherwise, and therefore have the luxury of staying true to my ideal of Mother Music, with no regard for any others acceptance of it. Free from judgement, other than my own.