Basically, the word that precedes the question marker (some form of mı/mi/mu/mü) -- the word to the left of it -- is the word the question is emphasizing.
In English, we would simply use inflection when speaking or use italics when writing. Emphasis on the word also implies a choice or alternative. A slightly different example might illustrate this better:
Eve mi yürüyor ? = Are you walking home (or to school or to the office or some place else)?
Eve yürüyor musun? = Are you walking (or running or driving or flying or using some other method to go) home?
Sen misin eve yürüyor? = Are you (or your friend or your mother or someone else) walking home?
I'm not really saying anything that contradicts what's already been posted, but sometimes wording something a different way from a native English-speaker perspective can clear things up.
Hope that helped.
This section of the thread is too nested to reply to you directly @AlexinNotTurkey, so I am replying to myself.
First of all, it has been a while since I've studied Turkish. So long, in fact, I had to read the following comments over a couple of times to understand what you and @cvictoria42 were getting at. After doing so, it appears that many of the examples I wrote would be incorrect due to my lack of vowel harmony, even though I so carefully bold it right up front in the beginning. Right? (I corrected them to what I believe they should be after reading your comments. If I missed something, let me know.)
I very vaguely remember exploring this construct in depth a long time ago, but have forgotten some of what I learned. Is it better Turkish to decline the verb for point of view (e.g., 1st, 2nd, 3rd/S,P) or decline the question marker? I can't seem to remember now, but I vaguely remember that one was better than the other.
Not Alex, but I think there are issues with the verb conjugation in your examples (the vowel harmony now looks okay).
"Eve mi yürüyor?" would be, "is (he/she/it) walking home?" If the subject is "sen," the question needs to be, "eve mi yürüyorsun?"
"Eve yürüyor musun?" looks okay.
"Sen misin eve yürüyor?" should be as Alex has it above: "sen mi eve yürüyorsun?" (As far as I understand, the verb's personal ending goes on the question particle if the question particle comes after the verb. If the question particle goes somewhere else, the verb needs to keep its personal ending.)