How to do a case study on achievement motivation? What are the questions I should ask the interviewees?

Questions should measure vision, determination and resilience.

When did you first start setting goals? Do you believe it’s important to make longterm plans (refer to study of Harvard grads and financial success as related to planning and goal setting)? How easily do you give-up when in a difficult situation?

Look for intrinsic motivations vs. extrinsic motivations. I think you will find that people with intrinsic motivations are more motivated. Meaning, people who are empowered by intrinsic motivations (ex. doing something to improve their situation) have a better chance of achieving something than someone who does it for an external motivation (ex. money). I know these are not questions, but it might give you a little more insight onto your topic.

There is a reason that many companies make the last interview a lunch interview. You can not really access a person when they have their guard up.

Same deal with an office interview. You already have the info about them on paper in front of you. Some people do not interview well but might be highly motivated once they have the job. Actually those people might be ten times better since they got the chance as opposed to those who can talk a big game.

Basically what you want to do is humanize yourself rather than have a list of questions. Talk about wife, kids, the weather or whatnot. How a person handles a bit of informality is the telling thing that will make your decision.