Learning new song a new way

[stewblack]

Well it's new to me anyway. Maybe you do it this way all the time, if so you could give me some tips on making the system work.
Usually I like to start knowing what the chords are and just feeling my way around the structure in an obvious root, fifth, octave type pattern or just roots to get said structure in my head. Then as I get under the skin of the song try out different ideas to different parts of the song till I've got basslines for each part of the song. Then I'll put them together keeping the best most fitting melodic parts and paring down the rest so as not to clutter the song.
This time however a mate has given me his demo and no chords (he doesn't know the names just plays what sounds nice to him) and asked me to accompany him at a gig next week. Great fun. I love playing with just a guitar as it's a totally different ball game from playing with a drummer. But as I've no idea what he's playing I've stuck the song on repeat and am literally just playing anything until I find something that sounds nice then trying to remember what I did. It's a weird unstructured way to write a line but I hope the result will therefore sound different from my usual style.