"Alright Mr Jones?"
"How's it going Mr Bradshaw?"
"What do you want to work here for?"
"Well, I reckon it's a wicked company and the job sounds cracking."
What strikes you as strange about the above? Of course, the formality and register of the conversation is all wrong. The best way I've found of dealing with formal and informal situations is to create a conversation similar to the above one, and ask students to rewrite it, substituting the words and expressions that are too informal with more formal equivalents.
This skill is tested in the Cambridge First Certificate exam, but can be made more humourous in a class situation, especially if you get students to act it out.