The Separate Amenities Act of 1953 codified much of apartheid into South African law. It made it legal to exclude people from public spaces and to create separate facilities that did not need to be of equal quality.
The purpose of an interview like yours is to get a firsthand, if anecdotal, picture of what it was like living under this particular law. Consider the following line of questioning.
Start off by asking some questions to establish who the person you are interviewing is. How old were they during apartheid? Ask some questions to establish who their family was and what socio-economic and racial communities they were part of. This will place the interview in a better context.
Next, you can ask for any specific stories of how this person was affected by these laws. It is always a good idea to ask the interviewee to share stories, not just facts. This gives the interview a more human feeling and makes it more relevant to others. It's also best to let the interviewee do most of the talking and just ask clarifying questions.
You should ask about how they felt about these laws. Also ask them if they know how others felt about it, like friends, family members, and neighbors. Understanding how apartheid was perceived at the time is great information to have.
You can end your interview with some questions that ask the interviewee to do some reflection on the topic. What advice do you have for South Africans today? What would you like people around the world to learn from your experience? Questions like these remind people that the lessons from the past are still relevant in the present.