Imagine you are watching two people have a conversation. You cannot hear the words they are saying, but there is still plenty to observe and deduce. Write what you see - focus on their movements, facial expressions, your impression of their emotions. Remember this activity next time you write dialogue.
An Ordinary Day
The challenge is to describe an ordinary moment in an ordinary day - getting up to make breakfast, catching a train, walking the dog. But your character is not you. Choose an attribute that would make your actions harder, or at least different - such as a small child acting without a parent's assistance, an elderly person moving with a sore hip, or an inconvenient superpower like uncontrollable spurts of invisibility. Use this exercise to develop your empathy and your characterisation skills.
When You Say Nothing At All
Watch this clip from 'Whose Line is it Anyway?' Note how exactly the same phrase is given different meanings dependent on the context and the delivery of the line by the actor. Your job in this activity is to write a conversation between two people where one of them has just one line - "I love you". Using everything around the dialogue, have this line be delivered in two completely different ways.
Take It to the Max
Choose one of the following adjectives and build a character around it. Take this description to the extreme, where it is the one attribute that defines your character and best determines how they will react in a situation. Then go here and select one of the simple scenarios. Put your character in that scenario and write away...
Adjective list: obedient, surly, polite, confident, merciful, vain, cheerful.