Here's what I do when playing guitar in a session.

I start by listening to the tune (if I don't know it) and anticipating the possible harmonic content. I don't play a solid 3 to the bar or 4 to the bar with full chords all the time, but play occasional chords with a passing bass line which complements both the rhythm and the melody - with plenty of gaps.

I don't do what Jack Campin so sarcastically describes in his post of 24 Mar 09 - 08:30 PM. My role model for this sort of harmonic work, many years ago, was (for example) Martin Carthy's very sparse accompaniment to Dave Swarbrick in the late '60s.

Fingerpicking, in the context of the sessions I attend, is a no-no. The massed volume generated by, say, 2 concertinas, 3 fiddles, 2 mandolins, bodhran, melodeon, whistles, Northumbrian small pipes and occasionally an accordion en masse means that, for all practical purposes, the guitar is essentially a back seat instrument in any case. But, whether back seat or not, it's nice to get it "right" in the context of the session, and feel that you're contributing to - rather than detracting from - the overall sound.