Social language

Do you know some expressions that you can use when it´s necessary to say you are only kidding and people shouldn´t take you too seriously? 

What about the opposite? What can you say if you want people to believe you and reinforce that you aren´t joking this time? 

I came across this table of "Being light x Being serious" when going through the pages of "New Routes". I suppose it can be useful to all of you. Check it out:

And now let´s imagine that Smudge and Monica could make use of one of these expressions after this childish argument: