I wouldn't try a walk up, especially if it's for something special, but I would check the app to see what's open last minute. For the heck of it, I just searched the website for tonight and there are options if you're flexible with time and location. Ex. If I were at WDW right now and wanted a character meal tonight for a party of 4, I could have a 7pm dinner at 1900 PF, 8:55 pm dinner at Chef Mickeys, 8pm dinner at Garden Grill, 8:25pm at Tusker...and so on. If I look at lunches for tomorrow, I have lots of options at Akershus, Garden Grill, Hollywood and Vine, etc. Obviously, this is all subject to change, but it's just showing that you'll probably find something open if you check for a last minute ADR.