Your characters are more than just people in your story. Your characters drive the plot forward. You shouldn't just be using them as mannequins. Characters should have their own individual personalities!
Make sure you describe your characters! Just like with scenery, you can't describe absolutely everything! However, pick the most important parts. Perhaps they wear an item of clothing that you could describe to tell us more about their personality? Perhaps your character is a very fashionable character and is never seen without her favourite Gucci glasses?
Finally, think about your use of speech! How would you character talk? Would they say, "Pardon? I'm sorry I didn't quite catch what you said." or are they more likely to say, "I haven't got any idea what you're going on about, mate!"
For more tips on how to use descriptive writing techniques, look at the section on descriptive writing.