When somebody asks, "Are you Serious?" respond by saying, "No, I'm Severus."
When somebody asks, "Seriously?" respond by saying, "No, Severusly.
When somebody say's, "You're a lunatic." say, "No, I'm a LUNAtic."
When somebody says, "You're being ridiculous." respond by saying, "No, I'm being Riddikulus."
But, when somebody says, "I'm Dead Serious." respond, "No, you're dead Severus." then have and emotional break down because Dobby is a free elf not a dead elf!