Well I mean you can change the rules in fantasy whenever you want. You can do that in any genre, really. But it has to be part of the plot. If you have (I'm going to use movies for a moment because it's easier to use time as a measurement rather than word count) an hour of a love story and then BAM out of nowhere aliens pour out of space all over the planet and invade and now it's a post-apocalyptic thriller, it's crazy and won't work.
But if it's a main part of the plot, it'll work. You can always change the rules. A war starts (common theme in fantasy) and the rules that once were in place are thrown out of whack.
Back to the main topic, one thing I do is make them me. The six main characters are all me, in a way. Before the "invasion", a theme in the book, they all had the jobs that I've dreamed of. They all have my anxiety, my anger, my fear, my lust for more knowledge, etc. I know them and make them react like they would because they're me. I know how they'll react and how they feel because it's everything that I've felt.