I can't give you any advice on spectator stuff, but I can give you my best guesses on where characters will be.

Again, these are GUESSES based off of what I saw in previous races.
STAGING AREA
There should be a character or two doing meet-ups near the DJ stage in the staging area before the races.
HALF (note that this is based off of what I remember from the
Princess Half, which is the same course as the WDW Half):
Expect a character(s) just after Mile Marker (MM) 1 on the left.
Except a character(s) just after MM2 on the left.
*Cross under MK entrance gates, pass MM3*
Another character after MM3
Enter MK, take a right into Tomorrowland. Should be a character out in Tomorrowland (Buzz Lightyear, Stitch, Chip and Dale or Pluto in space gear, etc.)
Go into Fantasyland - should be Alice in Wonderland characters out in front of the Mad Tea Party
Should be characters in front of the castle (Cinderella characters?)
Princess and the Frog characters in Liberty Square
Should be characters in Frontierland (Woody or Jessie, etc.)
Characters backstage as you exit the Magic Kingdom with parade floats
More characters in between MM7 and MM8, including characters in golf gear on the golf course
Once you get to MM9 you're heading back toward Epcot, so you'll pass the characters you saw in the first couple of miles, this time on your right
Sarge after MM10
Character after MM11
Character after MM12
*Into Epcot*
Should be a couple of characters in Epcot
Should be a couple of characters at the finish line
FULL (based off of what I saw in the 2012 WDW Full):
I think the portion above through the Magic Kingdom will be similar since the courses are the same through that point...so a few characters in the beginning with LOTS in the Magic Kingdom and characters at the golf course.
Not sure who will be in or around the WDW Speedway...Cars characters? No one?
Another meet on the backroads as you head to Animal Kingdom.
Characters backstage as you enter the Animal Kingdom
Jammin Jungle Minnie and Mickey after you exit Animal Kingdom
Not sure what to expect in WWoS, but this is where the mile 20 surprise is...and I am expecting/hoping for lots of rare characters.

Some characters backstage as you enter Hollywood Studios and a character before you leave the studios.
Characters around World Showcase (normal ones that are out everyday - Aurora and Marie in France, Mulan in China, etc.)
A character or two at the finish
...
Perhaps it was unnecessary for me to type all that out and just say that the characters are spread out through the course...but there you go!
Obviously you can see that there are a LOT in Magic Kingdom, so if the meet-ups are important to you (obviously they are to me) plan on that being one of your longest miles of the race.