Best piece of advice - DO NOT TRY TO GUESS THE DIFFICULTY OF THE QUESTIONS!!!.
My first time taking the GMAT, I got so flustered in the math section I mind literally stopped working. I would hit a question I thought was easy, and I would freak out thinking I had gotten the last one wrong. Don't do this to yourself! Second taking it, I was much more relaxed, and scored considerably better. (1st- 680 39q 44v, 2nd 720 48q 41v)
And to answer your question, you get random experimental questions dropped in anywhere during the test. The test does not adjust the difficulty level of the questions, nor does it adjust the difficulty after you answer it. Think about this - GMAC is testing the validity and difficulty of the given question. They might have an idea as to how hard a question is, but they have NO idea as to its validity or specific difficulty level. They are trying to determine these things, and that is why they are "experimental" questions.
