Specifically, I was thinking this in regards to Methos as I was watching the most excellent vid "And As for You" by killabeez, but it applies to Highlander in general. The passage of time. How must it be to not see someone for more than two thousand years, then they're back in your life? Are Immortal's brains different in how they perceive and deal with the passage of time? Would a century seems like a few months? Do they have a more vivid and complete memory system than we do? Methos says his earliest memories are barely there anymore, assuming he's telling the truth of course, so there must be some sort of max for them. So he has vivid memories of four thousand years ago, but not five?
Just for myself, trying to remember people that I was close to twenty five years ago, but haven't seen since, my memory is fairly hazy. But for Immortals, it seems that seeing someone millennia later is like they just said goodbye last week.