In an interview for a job that isn't what I studied in university, how do I prove my commitment to that career path to the interviewer? For example, a graduate of engineering expressing his commitment to work in accounting. It might be as a result of passion or as a result of what is available

This is going to be challenging. If I were the interviewer, I would be greatly interested in knowing why you want to switch and what makes you think you will be successful in a new career. For the switch, I would appreciate you being honest. As a smart interviewer, I would recognize that jobs might not be available in the field of your study, and I might even admire your ability to see that and your courage to make a switch. As to why you think you will be successful, I would want to know what sort of connections you have with this new field. Have you taken undergrad courses, are you now taking courses, do you understand the fundamental language of this new field, do you have any experience at all in this field? And even if I buy your story and am willing to train you, I would want to be convinced that you are a fast learner. You will have to come in with a very positive, confident and take charge attitude, but not an arrogant one.
Good luck - you will need it

Thanks a lot for the response. I will keep you updated on my response and their decision .

The interview was with PriceWaterhouseCoopers Nigeria and it was in four stages which I unfortunately did not know about;
A group discussion
An impromptu presentation
A Client briefing session
A one on one interview session

It was fun but I had to convince them at the one on one session why I was making that switch from engineering to accounting. I took them through my different business experiences - the one's I've worked with people and the one's I've worked on myself and told them that in all I've discovered that accounting tells a big story about a business - it's success, failure, financial standing and so on and I have an interest in understanding businesses and how they work, that's why I want to make the career switch.
I will know the end result by the end of April... Wish me luck

How do you think it went overall? Were you thrown by the different phases of the interview since you didn't know that would be the format? Any feedback on what they said or how they reacted to you?
All the best!!!

Thanks
I believe it went very well. When I heard the different phases of the interview, I was stunned. The other interviewees already knew what they were going to expect as friends had done it a day before. I asked for feedback and the interviewer at the one on one stage was like it's a process that will be added up together and she won't want to tell me I performed well or underperformed so as not to raise my hopes or dash them, I should wait till the end of April - a long wait .
It was fun though. I think it's the only interview I've thoroughly enjoyed.

I forgot to say that I hope you have sent them a thank you letter in the meantime telling them you appreciated their time and attention, that it was interesting getting to learn more about their company and some of the positves/ highlights you brought away from the interview. Do not make them forget you in the wave of interviewees - make yourself stand out in their memory.

Emmm, that's quite hard.
Interviewed by 5 different people and no e-mail addresses. I don't think they will mind especially in Nigeria. If they don't remember me, they should remember my stories.

Don't be lazy (ha ha). Call up the secretary and ask, even if it's just for a couple of the more senior interviewers.

Don't you think it's quite late now? It's over a week. I just saw the wisdom in it now