Hope you have a wonderful time.
I have been to DLP twice at Halloween. The first in 2016 and then again in 2017. 2017 the 25th anniversary took over and it wasn't as good as the year before. both times were at the very beginning of October so not sure if more is added later in the month and it may be busy if during holidays
In 2016 Maleficent was just the other side of the castle (cant miss if you walk through the castle itself)) there were thorns all round and smoke machines and she was there at certain times. nice and easy to queue and meet her, not too crowded.
In 2016 they also had a show on the main street side of the castle down to the right, there was a stage where the villains would do a show at set times and afterwards the are available in that area for autographs and pictures. we saw the ugly sisters and evil stepmother, jafar, the evil queen from snow white, Gaston, Frolo, and the villain from princess and the frog (cant think of his name) this was a little more chaotic as there's no real queue. we stood patiently and didn't push and i think the characters actually pick on the best behaved lol. the evil queen was very strict and keep saying no pushing , do not cross my circle. jack skellington was also near the phantom manor ride for pictures. there was an orderly queue system for this
in 2017 there was less Halloween decorations about, was focused on frontierland because of the 25th anniversary. however we did see captain hook, peter pan and Wendy near skull rock for pictures and these had queues.
the park guide/map which is available as you go through the ticket machines into the park. The map and schedule are separate, these will tell you who is available and when. we got to the peter pan meeting spot a little early to queue.
in terms of tips for the rest of the park, it may be wise to take a travel kettle with you to save hiring one. can get them for about £10 in Argos and get the UHT milks in b&m or ebay etc. we do this every time and also take coffee sachets, hot chocs and tea bags for the evenings and so we can have a cuppa as we get ready in the morning. for the little ones we take one of those Robinson squash things that you can add to a bottle of water as i don't like fizzy drinks and the only non-fizzy bottled drinks in the parks are water or ice tea. my nephew had cola with his lunch and dinner but in the day it was handy to buy a bottle of water (that you can refill at water fountains around the park) and add squash. we also take snacks with us like cereal bars or veggie crisps because although there's lots of sweets and candy floss and donuts about you don't always fancy something sweet and it saves money (i sound stingy but more money for souvenirs if you save money on food
and handy if stuck in a queue for a while.
If you are going by train i recommend the Disney express extra which means you can drop your bags off at the top floor of the train station and head off into the park, you can then collect at your hotel later which saves a bit of time when you first arrive, straight off to the magic rather than trekking to hotel and then back to park. this can be paid for when you get there at the DE desk but they only accept card no cash. on departure day you can drop bags off at the hotel and they are waiting at the station for you later. think the collection point is the same floor as the euro star gate if i remember correctly.