Question about phd interview gone wrong Recently I had an interview for a phd in computer science. In the application I was able to choose multiple thematic categories, so I decided to apply for complex event processing, as well as robotics. During the interview I was asked to explain the reasons that I decided to choose these categories. I tried to explain that I am interested in applying AI techniques in data stemming from multiple heterogenous sources and that I believed that both of these categories would give me such an opportunity. Although the methods that I would use to do this as well as the application I would develop would be different for each thematic category. However I believed, that this would not be a problem. The interviewers though seemed dissatisfied with my views though. One of them suggested that I lean more towards the complex event processing thematic category. They asked me then whether I would be interested in applying complex event processing in medical applications (since I have an MSc in Biomedical engineering) or some other area of application such as finance. I replied, that I am more interested in exploring the methodologies involved than the actual application and that both the cases of finance and medical data excite me. However I think that the interviewers were possibly dissatisfied with my answers as I did not seem to come to a closure regarding my decisions. They then replied that a PhD is a process of researching a subject in depth... They also mentioned, that although in my CV I had done several things in the past related to computer science (such as jobs, conference papers and journals) they all were quite different from each other. This could possibly indicate that I was more interested in learning several things instead of giving special emphasis in one area... I replied to this by saying that the fact that I took advanage of some opportunities in the past did not necesserily mean I would not be willing and motivated enough in researching a subject for a long time.

However I feel dissatisfied with the responses I gave in this interview. The reason is that I realised that if I had a gun pointed at my head in order to choose a topic, I would choose the application of complex event processing in finance data. So I want to ask the following:

I am able to talk to one of my interviewers. Would it be a good idea to tell him that I realised that if had to choose only one thing, that would be the application of complex event processing in finance data and I would forget about robotics or the medical modality as they only seem nice alternatives (with the exact words described above). (I have no previous experience though in these areas ... but I feel really motivated in researching this topic)
I think you clearly sensed the concern that the interviewers might have with a candidate who is capable but seemingly unfocused. PhD completion rates are fairly low and one contributing factor is the motivational slump that occurs after a couple of years with very little light at the end of the tunnel. One way (that committees try) to avoid these unfortunate scenarios is to select candidates that are intrinsically motivated by their topic: whose passion for the subject itself will sustain them through the bleak slog that is methodology.

The problem is basically that, as you say, if someone held a gun to your head you could commit, but your advisor is not allowed nor obliged to do that. You need to decide independently whether you are honestly, deeply motivated by the desire to tackle hard problems (and make very little progress) in your subfield.

If it is the case that complex event processing in finance really is the field that will get you through years of low pay and minimal recognition, then you could send a follow up email along the lines of "Thank you for your time. I appreciate your consideration. Upon reflecting on our interview the other day, I feel I may not have appropriately conveyed my interest in SUBFIELD_X, which I think is important for VALID_REASON_Y" However, you should not be terribly surprised if this doesn't dislodge a negative opinion.

Edit: I should of course add that in many, many cases the specific (METHODOLOGY x SUBFIELD) that you are deeply passionate about today, will not ultimately be the one in which you complete a thesis. This is okay, as long as the passion carries over.