Have your characters put on a costume and do a masquerade.
Anytime.
I find it most useful during the discovery phase of writing, before most things are quite set.
Stanislavski described an exercise called The Masquerade. This exercise required the full embodiment of an actor into a character. The goal, though, was to have the character be specific instead of general. To avoid archetypes, the actor was supposed to have very specific traits that seemed to differentiate their policeman from the policemen that stood around them. How can we, for instance, know which detective at the convention is Sherlock Holmes simply by minute behaviors? The trick, or course, is that these aren't supposed to be planned out in advance, but arise quite naturally "in the moment."
Letting your subconscious create unconscious physical ticks via external characterization is but one part of the method Stanislavski wished to teach. It wasn't about extreme levels of control, but about harnessing what one could from the superior subconscious. This exercise asks your subconscious to work a little bit. It also asks your characters to do something strange: pretend.
Now that you've spent some time in someone's shoes, think about what -- if anything -- you learned about your characters &/or your play by having them pretend and dress up? Did the dialogue change? The interactions? The stage directions? The pace? Something else? Was any of it better or did it simply solidify how right some other option truly is? Proceed without the mask, Anon.