Some of the Villians that were placed around the hub (Tremaines, Lotso, Gaston, Jafar) were out all throughout the night except when the Villians Dance Mix & Mingle was on. They had shorter waits and were easier to get to.
The Villians in the show (Malificient, Cruella, Captain Hook, Dr Facillier, Snow Queen, Queen of Hearts) all came down from the stage directly after the show was over, and did meet & greets (all really quick, just pictures, no autographs) for a short period of time before they went backstage to prepare for the next show. You could get one, maybe two in each show time. The best thing to do, is watch the show, and be as close to the front as possible, make a beeline for the line you most want, and then take note of where everyone else is, and be in place for their line after the next show to meet them.
Character interactions at MNSSHP are fantastic, and be on the lookout for characters everywhere, and all kinds, the lines for some will be really long, but others are nice and short. Also keep your eye on the disboards for information about who is where (I THINK, that on the map, it just has the little hand symbol, meaning there's a character there, but they won't tell you who) Some of them are easy to guess (ex: Alice & the Hatter are the ones at the tea cups) while some of them aren't.