I have never seen John Smith. Each park has character locations and greeting times, and you'll see them listed on the maps with a big white pointing glove marker.
That said:
In MK:
The town square will have classic and random characters. We've seen Pluto, Goofy, Donald, the Chipmunks, Daisy, Pinnochio, and some random bears. In Tomorrowland, you may find Stitch and Buzz Lightyear. In the Judges' Tent in Toontown, you'll find three princesses and three fairies, in two separate lines with long waits. Mickey and Minnie usually greet guests in their houses/gardens. Alice and the Mad Hatter can be found near the teacups. Pooh and friends are near their playground in Fantasyland. Ariel is in her grotto. You may run into Aurora, Prince Phillip, the Fairy Godmother, Mary Poppins, the wicked Stepmother, and the stepsisters around the back side of the castle. Belle reads stories in her garden to the right of the castle. Jack Sparrow, Captain Hook, Smee, Peter Pan, Wendy Darling, Jasmine, and Aladdin greet guests along the path at Adventureland. Pocahontas used to be along there, but that's now a smoking area. And in the back of Frontierland, we have seen Goofy and Donald. Tweedledee and Tweedledum sometimes have come out in the area in front of the castle. I am sure I am missing some.
At Epcot:
In Future World, there's an indoor signing area with classic characters. In the World Showcase, Donald from the Three Caballeros is to the right of Mexico, Mulan and Mushu are at China, Snow White and sometimes random dwarves are near Germany, Aladdin is near Morocco (they said the Genie had 'retired' last time we were there), Belle and the Beast are sometimes near France, Winnie the Pooh and Mary Poppins (and sometimes other characters from that movie) are near England, and the Brother Bears are near Canada.
HS: Random characters appear near the entrance. Playhouse Disney characters sign near the PD show. Incredibles, Frozone, and Pixar characters, plus Mickey as the Sorcerer's Apprentice, sign in the Magic of Animation attraction. Toy Story characters are near
Pixar Place. Random and classic characters sign around the hat. Cars and Power Rangers are at the back left corner.
In AK, there's a whole trail with characters. Classic characters, Lilo & Stitch. Characters can also ben found outside the entrance (Jungle book, etc.). We've met Rafiki, Pocahontas, and Stanley (dating ourselves) at Rafiki's Planet Watch. I hear Jiminy Cricket is back there now. Pooh & Friends have a landing off of Discovery Island.
All of that said, if there's a character you really want to meet, you can call Guest Services and ask if and where they are available that day. One trip, we went the whole trip and didn't find Cinderella. We didn't realize that she also greeted guests at the Judge's Tent, but Guest Services pointed us there.
