To give you a bit more perspective on why people keep saying you really can't pin down your interests before you go in, as a final year student at the moment.
As an applicant, I got interested in doing medicine purely because I watched Scrubs and House on TV - and that's the truth. I had had a fair bit of contact with doctors growing up (through myself being ill as a child, and family members being ill when I was growing up), but it didn't really inspire me to become one or anything. I was always scientifically-minded, I loved a bit of problem-solving, wanted to be the "detective" figure, and really did want to help people. So following in the footsteps of J.D. and House seemed a good way to go. This is how I answered the "why medicine?" question, while like ecolier said, leaving out the stuff about TV hahaha.
I was most interested in: A&E, medical specialties (like cardiology, respiratory medicine, infectious disease, neurology and nephrology).
I felt neutrally about: surgery, of all kinds, anaesthetics/ICU
I thought I'd hate: dermatology, psychiatry, GP (this is where I did all of my work experience), and musculoskeletal medicine
Obviously I had no idea what working in any of these was like.
Now as a final year student
I absolutely LOVE: Psychiatry - turns out that psychiatric illness is completely fascinating; it's a patient population I can empathise very heavily with and I find building a therapeutic alliance with them rewarding; and you can make a big difference in their quality of life. Good work-life balance. Good understanding of a niche area of medicine. Unique set of skills. Good balance of ward and clinic work.
GP - huge variety of presentations requiring breadth of knowledge; lots of opportunities to manage undifferentiated illness (detective!!!); lots of things for you to take a special interest in (I can do my bit of psychiatry and bit of MSK and bit of derm right here!); you get to make good long-lasting relationships with your patients; great work-life balance; and I actually turned out to really like sitting in a clinic room all day as opposed to sitting about a ward repeating the exact same process on every patient 40x a day.
Dermatology - interesting pathology; impactful management; great lifestyle; $$$$$$$.
A&E - I ended up being right about this one. I'm a complete A&E junkie. I love working with acute illness. It's the most interesting part of medicine. And I literally get to play as a detective all day every day. Your management has direct impact on patients; you get to a great mix of brainy stuff and hands-on stuff. Only bad side is you often don't know what happens to your patients next (short contact time) and your lifestyle ****ing SUCKS. That's why I probs won't end up doing this; it has to really be the only thing keeping you going in life to be able to put up with the shitey shift pattern.
Acute Medicine - Didn't consider this before starting med school. Same reasons I like A&E. Acute medicine is kinda like A&E except it's the period immediately following that first four hours, which offers its own unique opportunities, skillsets and challenges which is rewarding in a similar but different way.
Things I feel neutrally about:
Anaesthetics/ICU - I've always been a physiology nerd, so anaesthetics fits me well. It's got lots of practical sides to it. I like the idea of being the proverbial master of the airway. And it's kinda cool to be a literal life-saver. ICU interesting as it's like a mix between anaesthetics and acute medicine. But I dunno if I like this one enough to do it as a career, but I did really enjoy it.
Radiology - their life sounds so god damn lit.
Things I absolutely hate:
Surgery - I never imagined how tedious, boring and mind-numbing operations actually were. God forbid, I'll insist on having any procedure I have to have done on me under GA just so I never have to spend a single waking moment inside a theatre beside a surgeon ever again. It's a shame, because the diagnosis and medical management of surgical problems is actually pretty interesting. I don't think I'll mind working surgical wards too much as an F1/F2. But surgery itself is so mindnumbingly dull that I've avoided it as much as possible during medical school, and I'll continue doing so after I graduate.
Most medical specialties - interesting pathologies, interesting management plans, boring job. Endlessly rounding all day and then fiddling with medications for your chronic population? No thanks. Most of the ones I thought I was interested in pre-medical school I actually ended up hating so much in medical school. Inc. neurology (sorry écolier!
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Paediatrics - *****y lifestyle; kids suck.
O&G - how anyone likes it is beyond me.
So you can see how my actual interests ended up turning out almost exactly the opposite from what I thought they'd be when I went into it!! So be careful not to judge books by their cover: everyone else is right that you have no way of figuring out what you're actually interested in until you've worked the jobs.