Most non-classical guitarists really are their own arrangers(this really means everyone who isn't playing directly from a written musical arrangement)--as you can tell from Eve's post(others discussed it too, so don't feel overlooked) the guitarist has to decide which sort of rhythm pattern to use, what (if any) bass lines to play, what to fill in between verses, which chord voicings to use, whether to play a moving or static bass line, where to use melodic fills, and exactly what that should be--and on and on--

A lot of players don't really even realize quite that they are making choices, the always play the same walkdown when a certain chord change occurs, or always play the same turn-around in a blues tune--