A few other tips I like, curtesy of the Timbuktoons.com article you can find here, include going deep, not forgetting to record the incidentals, and always doing multiple takes. When you give an actor a line to read, it's going to be almost meaningless if they don't know the context. You need to give their character a backstory - as goofy as it sounds - and help the actor be able to identify with their character. Even if they're playing something that they have nothing in common with, a good director should be able to find that common ground and make actors see their characters in a different light.
Also, from a more technical standpoint, it's important to keep the mic running a bit both before and after each take, to catch some stuff that you may otherwise miss and want to keep in. At the same time, one take is never sufficient to know whether or not you have the content that you want (plus it's easier to lose), so you want to have your actors read their lines a variety of ways to make sure that you've covered all your bases.
So long story short, the next time you find yourself in need of some casual voice actors, make them do some weird things help them both A) Get their inflection right and B) get them in character. Canon and Diana (or whoever is directing producing our group's film) take note, and go cast an awesome blender.