The PP is right..Just to add a couple of examples, I am always asked for the full names, birth dates and professions of both my parents and all 4 of my grandparents so perhaps you should have this information to hand just in case ;-) My cell phone is searched each time I enter Israel/West Bank and all Islamic-sounding/Iranian/Turkish/Arab names are noted down on a paper before the security staff ask for a full and very detailed account of exactly how I know each person, where we met, the person's profession, marital status and current location and someone types all the information into a computer whilst I am saying it... So if any such names are stored in your cell phone or diary then please be ready to go into a lot of detail about each person - even if you say you haven't seen the people for like 5 years or something it doesn't make any difference...To save time, whenever I enter now, I "change" some of the names: For example "Mohammad" will become "Michael" and "Saeed" will become "Simon" and a couple of the digits are changed around - they are just innocent law-abiding friends but the suspicion their NAMES attract is almost unreal!! Last time my cell phone was "searched" the process still took about 2 hours.
You will almost certainly be asked for the details of where you plan to stay...If it's a hostel/hotel then be ready to show your booking confirmation documents. If you plan to stay in a private household (particularly an Arab household) then you should be ready to give the full life-story of the family in question - full names, professions, how/where/why you met, how you stay in contact, their email addresses, phone numbers, workplace addresses, who else visits the house etc...You get the picture!
Another thing, I have nearly always travelled there with Turkish Airlines and the security staff seem to take particular interest in this..."why Turkish Airlines?" - "why not Air France or KLM...?" for example. They always ask "tell us what you did in Istanbul before boarding the plane (like they CARE that I went for a coffee in the airport or browsed in the Duty Free shops!)...did you speak to/meet anyone?", "did you have anything to eat, if so what was it?", "what language did you speak in Istanbul airport?", "what time was your check-in?", "did you speak to any other passengers during the flight?"...etc
As the PP says it's best to be honest even though the repetitive nature of the questions can get tiring.