In a sense, the answer is in the expression 'semi-structured'. So, a completely structured interview is a *conversation where all the questions are pre-scripted* and very often the answers are *closed answers*, so like an option 1 from 5 or a closed question like 'Yes' / 'No'.
And if you're going to do that kind of interview, really it's very much like a *survey*, and you might almost do it – it might be better as a written survey rather than as an interview. At the other end of the spectrum, you just have a *conversation, a chat with somebody*, which might or might not be on a particular topic or might range across a whole spread of topics and cover all sorts of material not necessarily with any particular structure and not with a particularly obvious purpose to it.
*Semi-structured lie between these extremes*, i.e. there is some kind of structure to it; you typically have a *topic guide* or a *semi-structured interview script* that determines the kinds of topics that you're going through and an initial planned order with which you'll cover those topics. But it's also free form in that people are welcome to give long answers, like the answers I'm giving you now.
And you can elaborate on questions; so, if somebody says something particularly interesting that you hadn't anticipated, you might follow up on that. And indeed sometimes people answer the topic that you'd thought of as being the fifth topic – they might introduce that when you thought you were still talking about the second topic; and so, it kind of *free-flows* to some degree and feels natural. But as an interviewer, you have an agenda;
you have a set of topics that you want to cover; you perhaps even have some detailed plans of how you're going to ask some of the questions so that you get through the topics that *you* want to cover even if participants maybe go off topic at times or – you know – divert into other things. So, that's roughly what a semi-structured interview is. It's a *conversation*, but it has a set of *topics* that you want to cover
and is *organized* such that you're reasonably confident that you will cover those topics, but perhaps in a way that is responding to the participant and their interests and to some extent their agenda as well, as well as that of the interviewer, but it's not a completely fixed structure where you're not allowed to deviate from what you've planned ahead of time. In HCI, I think they're the most common form of interview because
we typically have an agenda; we're concerned with finding out from users or from potential users about the requirements for future designs or about the way that people do their work so that we can design new technologies to support that work or that activity better. Or maybe there's already an existing technology that you're testing,
and so you want to know specific things about what works well, what doesn't, and so you have an agenda as an interviewer; it's not just a casual conversation. But you do want to know a set of things about that thing. But you also want to hear what the user has to say; you want to listen to them, especially when they say things that are *unexpected*,
as well as finding out the answers to questions that you've asked. So, as interviews go, they are – I would say – the most common form used in HCI.