Was thinking about this on today after I had worked with all three of my bands this weekend (Yeah rubbery legs last night loading up the car). Playing covers is like acting in a one act play. Everyone knows the script including the audience and you are expected to produce enough of the nostalgic resonance to have them recognize it. During the "cover play" the guitarist and/or singer(s) usually have the lead roles and the supporting cast members all say their lines when queued. Getting the emotional tone set, continuing with the one act analogy, is the hardest part in playing with new bands or learning new songs, or sensing when it is okay to improvise. IMO, the most boring bands are the ones that try to reproduce the cover, and many guitarists fall into the 'idol' trap of trying to pull off the same licks, distortion and body movements as the original artist. S**t, they don't even play it like that anymore. Bass tracks on recordings are often simplified to make it easy to hear them on a blown '68 Beetle car speaker (Don't pepper me with Claypool or Flea references, I'm talking the Eagles, STP). I don't often play these lines verbatim, unless it is the hook of the song (Super Freak, Brick House). What I try to do is listen to what the other 'actors' are doing and react, prompt and looks for holes where I can punch an appropriate 'tasty' line albeit slap or arpeggio. What do you do?