To answer your questions, officially, walk in appointments are never allowed (for interviews - document updates are always walk ins).

Same day, they may be able to accomodate you earlier, though YMMV. During my last renewal, which was almost 5 yeas ago since I just put in my renewal a couple of days ago, I scheduled an appointment in the evening at YYZ, but had to be at the airport late morning to drop someone off anyway, so walked in to ask. They said Canada could see me right away, but would have to wait for US side to complete. Believe it took somewhere over 90 minutes, but they did. Not sure it was better than just going back later, but it worked.