I think I agree with FF5 on this. I disagree that feigning interest is right. FF5's point is this: you don't need to feign interest or even like what you're doing on a rotation, and you don't have to pretend or say you do. But you need to recognize *why* you're there and more importantly *what you're supposed to get out of it* and convey that, rather than feigned interest, instead. Let's use surgery as the classic example: as a student its OK to be like "this is REALLY not for me, I appreciate you guys that do this but I don't enjoy it. But I definitely know why I'm here and why I need to learn about it so I understand proper referrals, diagnostic work-up that can be done prior to it getting to a surgeon, and steps I can do to make your life and my patients life better and expedite/improve their care from my end. In fact, Mr. attending/resident, is there anything I'm not thinking of that you can teach me that would help me do that? Something else I can study or learn more about?" You don't have to say that with a smile. In fact, you can say that being distressed AF because surgery is really stressing you out and be totally honest about that. But if you're approaching it with the same energy/attention/desire to improve that you do with something you really like, that's totally fine.
That's a high pass every time at minimum for me, and usually honors if they then follow through.