At 1:09 into Darkest Hour Winston Churchill (Gary Oldman) says (yells):
You cannot reason with a tiger when your head is in its mouth
Is this an actual quote (I cannot find it), or just the creativity of the script writer?
This is an invention of the movie. The quote is a type of "ancient Chinese proverb", a saying that has been around so much time that many people have said it at least once.
Also on Amazon you can find this quote with this statement "The quote comes from Winston Churchill in "Darkest Hour", a movie that provides a look..."