There is good advice above. I also want to add: when someone is kind, especially older people (I’m 56), we ask you what you want to do with the expectation that the sky is the limit, and we enjoy hearing people with options talk about what they’re passionate about. We don’t expect you to have it all figured out. So if you told me in a social setting, “I don’t know. I might decide to fly helicopters, but I also like studying fashion, and I adore basset hounds so I would love to be a vet” — that is completely NORMAL sounding to me. 🙂 If someone, on the other hand, is judging you because you don’t have your career trajectory mapped out at the age of 24 during a social conversation, forget ’em.
Of course, I know the job interview situation is different. I agree with the advice above. I have to write things out at times so I don’t deviate from the script.
Be yourself, and your enthusiasm will shine. Good luck!