Three ways to differentiate your characters

A couple of months ago I wrote about multiple POVs, were they a good idea or not. At the time I mentioned that, however many characters you have, you want your reader to remember them and be able to identify them instantly without their names. Apart from the physical description [another topic for the future!], how do you differentiate your characters?

My research including asking Twitter’s #writingcommunity, brought forth some great tips and hints to find ways to differentiate your characters. H...