First, sit down and think about the character.. Don’t think about motivation or their past right now – just think about externals. (At this point you should have already come up with a back story for them) How they walk, how they stand, think about what they wear – if you have an outfit like theirs, put it on. If you’ve decided your character wears six inch, put on the highest heels you own. Basically,try to become as much like them as is physically possible for you.
Then, once you’re in character, go to the shop. Use the route they would use, react like they would react, buy the food they would buy. Behave just like they would.
Once you get home, slip that character off. You’ll notice two things. First, that you are exhausted. Unless your character is the most relaxed person in the entire world, they’ll use and tense up their body in a different way to you. That means you’ll have been using muscles you’re not used to using and it’s tiring. It’s all very well saying ‘walk a mile in someone else’s shoes’ but no-one ever points out how damn painful that is.
Second, it’ll give you a whole new perspective on why and how your character develops. If they wear six inch heels, you’ll have a whole new understanding of the restrictions that can cause. If they carry a heavy bag with them everywhere, you’ll understand how awkward that can be. If they hunch into their jacket all the time, you’ll understand how that gives you backache, or if they swing their arms and stride out into the world, you’ll see how people look at you differently.
And understanding that, understanding their physical limitations and freedoms can give you an insight into character. Of course someone’s in a bad mood if their feet always hurt.
I once tried this exercise copying a friend of mine. What I hadn’t realised before I tried it was that the way she walked meant her back always ached – no wonder she always wanted to sit down. And interestingly, when I was being her, I lost my slight phobia of dogs.
It’s one of the best ways to explore your character – be them for a while. Once you’ve done that, its so much easier to understand them.