If they are coming from the other direction then I'll nod, wave, or say 'Morning' or somefink. If we are in the same direction then I'll definatley say hello, nice wheels (if they are nice), etc but more often than not it will not lead to a conversation.
If someone is broken down at the side of the road ten I will stop to see if I can help. Did this a couple of weeks back to some guy who was stranded with a nasty p***ture....so offered up a spare tube I had in my pouch. We were only 10 miles from home and rode back together....and had a great chat. He tried to offer up cash for the tube.....but I refused with a "call it karma....the only payment I will accept is that next time you see someone stranded at the side of the road, ya help them out'. Said he definatley would.