Why do people sometimes think for a very long time?

I played at a tournament over the weekend, and in the third round I got paired up against a guy rated 1800. I had black, and the start was 1. e4...c6 2. d4...d5

For his third move, he sat there for thirty minutes before playing 3. exd5. The whole time, he was staring directly at the pawns in the middle of the board.

I talked to him after the game, and he seemed very sharp. I asked him what he spent the thirty minutes thinking about, and all he said was, "I was trying to decide which variation I wanted to play."

Is this behaviour weird/normal/absurd?  

sure, it's normal.  thousands of tournament games are lost in the opening.  the idea for such long controls is to give us the time to make the very best decision possible. in classical controls, each side has 3 hours to think, so using 30 minutes on one move isn't unheard of at all.  i've done it.  :)

I think he was trying a psych job.

Completely normal.  I don't think I've ever spent that much time on a move so early unless I didn't know the opening we were playing, but it doesn't sound weird or unusual (unless, of course, it was a G/60 or faster).

There's a story of a GM who took 30 minutes or so over his first move with the white pieces.  When asked why he said something like "There's just so many choices"

And I read a story of one who wound up thinking a half hour on his second move (as white, I believe) -- IIRC black had initiated a defense he wasn't expecting, and the GM spent all that time trying to figure out how to get out of it as quickly as possible.

I can top that. The author of a book on the Vienna opening for white (I think it was someone named Weaver Adams) once sat thinking for 50 minutes on his second move:

2. Nc3

Whew! Good thing he had written a book on it, imagine if he was apporaching this position from scratch!

Heck you might be playing that game for 4 or 5 hours then if you mess up the opening, you are in for a tough day at the office. Most OTB tournament  games are LOST in the first 10 moves as stated above, people taking 30 minutes on a move is common with longer time controls as early mistakes generally in OTB tourny's are punished swiftly and decisively!

 I agree you need to be careful, but it is easy to go overboard on it. A 30 minute think on a position well known to you and to theory is a gift to the opponent. I find it hard to imagine anything really productive is happening past the first 60 seconds or so.

"I find it hard to imagine anything really productive is happening past the first 60 seconds or so."

A bad habit, especially in OTB, unless you are in time trouble. Its always better to take your time and not make a hasty move, you may see something better or you may not make that blunder. 

Here's anand thinking for almost two minutes on move 4. Of a blitz game.

And he wins.

http://www.youtube.com/watch?v=BwcONHV1csI

How could anybody not like him?

I like Anand too but maybe he was having a flashback to this game.

http://www.chessgames.com/perl/chessgame?gid=1018015

they are probably piping it , and nodding off for a few minutes

Ouch

When asked what he was thinking about during a lengthy pause Tal answered---Well, I was intending to sacrifice a knight. The sacrifice was not altogether obvious, and there was a large number of possible variations, but when I conciously begin to work through them, I found, to my horror, that nothing would come of it. Ideas piled up one after another. As a result my head became filled with a chaotic pile of all sorts of moves, and the famous "tree of variations" from which trainers reccommend you cut off the small branches, in this case spread with unbelievable rapidity. Then suddenly I remembered the classic couplet by Korney Ivanovich Chuvkosky:

Oh what a difficult job it was

to drag out of the marsh the hippopotamus

I dont know from what associations this hippopotamus  got onto the chessboard, but although the spectators were convinced that I was continuing to study the position, I, despite my humanitarian education, was trying to work out: just how would you drag a hippopotamus out of a marsh? I remember how jacks figured in my thoughts, as well as levers, helicopters, and even a rope ladder. After a lenghty consideration I admitted defeat as an engineer, and thought spitefully: 'Well, let it drown!'  Suddenly the hippopotamus disappeared, went from the chessboard just as he had come on- of his own accord!  Straight away the position did not appear so complicated. I somehow realized that it was not possible to calculate all the variations, and that the knight sacrifice was, by its very nature, purely intuitive. Since it promised an interesting game , I could not refrain from making it.

The following day, it was with pleasure that I read in the paper how Mikhail Tal, after carefully thinking over the position for 40 minutes, made an accurately calculated piece sacrifice.  

How bout Paulson thinking for 1 full hour after a Morphy Queen Sac

Both Bronstein and Rubinstein were both noted for taking a long time on their first moves.   One thought is they are visualizing the whole variation they want to play.  Rubinstein was trying to remember games his opponent had played.  If you are visualizing a complicated variation and suddenly go blank on a key move, then you could change to another variation. 

Morphy and Paulsen were playing in the days when there was no time control and thinking over an hour on a critical move was common.

still, it took morphy only 12 minutes to find and calculate it, it should not have taken Paulsen that long

 I dont think you read the first part of my post. I was referring to opening positions known both to you and to theory. For example, you are a sicilian player and white makes the move 1. e4. What useful thoughts do you have that take longer than 60 seconds here? Especially considering you have limited time and undoubtedly positions will arise later that are NOT known to theory, thats when you will really miss the wasted time earlier.

See also http://www.chess.com/forum/view/community/everything-that-you-do-or-say-will-be-misconstrued-on-a-chess-site