I have met Martin before, he's a real down to earth guy. He studied in a University near me for a semester a couple of years ago (his mother is Irish and came from that area) and they gave him an honourary degree I believe...
Anyhow my friend is an actor and at the time he invited Martin to come to the University acting school and he came in and spoke to them for an hour and told them lots of cool stuff...but the coolest thing was he did a table quiz with the guys a few nights later.
Guys texted me 'cos I didn't believe them so I went into the bar and sure enough Martin Sheen sitting down with 4 of the lads sipping on bottled water while the lads were downing pints. He paid for their booze all night and explained how he didn't drink anymore and was teetotal.
Funniest thing was...no lie...one of the questions in the quiz was "name the father/son duo who starred in Wall Street together" and he whipped out his passport and gave his real family name : (taken from Wiki) Ramón Antonio Gerardo Estévez....and they got the question wrong much to his amusement. Cool story, bro.
edit: Martin did actually attend lectures btw. I used to see him all the time in the hallways just like any other student. Nobody hassled him really, he had a keen interest in history I believe.