God forbid. The wind might change (c)
One more question about "Easy Virtue" movie lines, if I may. There is a following conversation taking place:

Mrs. Whittaker (very disapprovingly): You're smiling, Jim.
Mr. Whittaker: God forbid. The wind might change.

Could you please tell me what exactly does Mr. Whittaker's phrase mean? Is it an ironical "Of course I didn't" or a scoffing "Oh what a catastrophe"?
I tried to google it but I guess it's too obvious to attract anybody's attention so I haven't found anything of use.

*If there is a context needed, there is a link to this moment: https://youtu.be/JRJ8vghpmaU?t=2253 (37:45). Sorry, the video quality is extremely low, but that's the only freely available video containing this conversation that I've managed to find on Youtube.

Question

God forbid. The wind might change (c)
One more question about "Easy Virtue" movie lines, if I may. There is a following conversation taking place:

Mrs. Whittaker (very disapprovingly): You're smiling, Jim.
Mr. Whittaker: God forbid. The wind might change.

Could you please tell me what exactly does Mr. Whittaker's phrase mean? Is it an ironical "Of course I didn't" or a scoffing "Oh what a catastrophe"?
I tried to google it but I guess it's too obvious to attract anybody's attention so I haven't found anything of use.

*If there is a context needed, there is a link to this moment: https://youtu.be/JRJ8vghpmaU?t=2253 (37:45). Sorry, the video quality is extremely low, but that's the only freely available video containing this conversation that I've managed to find on Youtube.

Answer

The phrase "the wind might change"  refers to something adults used to say to  children: That if you are frowning/scowling/making a bad face and the wind changes direction your face will be stuck in that expression forever! 
"God forbid" is a way of saying "I really hope this doesn't happen" 
I don't know the movie and didn't watch the video you linked, but I imagine it is an ironical use of both phrases.
I hope that helps