If improvising dialogue in a commercial casting, ALWAYS KEEP POSITIVE - nobody wants to see a sofa advert with a parent telling off a kid or people complaining about something. Negativity, moaning and arguments tend to be the easiest type of improv. so a lot of actors incorrectly go down that road. 

Important to remember that improvisation doesn’t have to make sense. If they have specifically asked to see you perform a certain emotion, make sure you get there! If you are improvising with someone who just won’t lead the scene in the way you need for you to organically get there, then just make something up out of context to ensure you show that emotion. The CD will understand that the other actor wasn’t stepping up and appreciate that you took the steps to show what they wanted. 

i.e. if they have asked you to get mad when the other actor won’t do what you want, but the other actor keeps agreeing in the scene, just pretend they have said no and get angry anyway. Hopefully the other actor will twig they missed the note and roll with you.