Well, first off i have to say it all depends on song, arrangement etc. When that is said, i usually feel more liberated when playing latinstyle music... Even though the main bass line in most cases plays I and V, there's usually more space to do short figures fill out whenever the melody is having a break, or to comment on someone elses solo's.
As previously mentioned this depends on size of band, style etc.
In general chord notes (especially thirds) are well put into your play. Also very much scale figures og sometimes chromatic figures do sound very neat, mostly when played in 8'ths and 16'ths and ofcourse put into the right context. I know this isnt very specific, but you really just gotta try and then you'll figure out what works afterwards.
About straight life. I think of the two chord changes as independent, meaning you should move the mixolydian scale from being rooted in F, to Eb, each time. Ofcourse this is open for discussion, and lots of other options, but if you do as i would, especially the seventh note is well put into play, again dependent on the specific arrangement one plays it in.
I hope this helps you, if youd like me to i could transcribe some figures that i would be using in play to examplify, but i am not sure whether it would help.
You should endeavour to "play what you think", but i am aware that isnt as easy as it sounds.
Over and out.
