6K Man
Bureaucrat
IIRC a beg will always be successful against a friendly/pleased AI if 1) you haven't begged recently, 2) you don't ask for too much, and 3) your total begs haven't exceeded some value. So if you want to beg to prevent a DOW by an AI who plots at pleased (and get a 10-turn peace treaty), it makes sense to beg for something small - say, 10 gold.
You can't beg from an AI whose attitude is Cautious or worse.
I haven't had any luck demanding stuff from an AI who was plotting against me. I suppose once they feel they're strong enough to attack me, they feel able to tell me to get lost when I demand things. What does work during a war is demanding things from neutrals, so they can't be bribed into the war or vassal to the AI you're fighting.
You can't beg from an AI whose attitude is Cautious or worse.
I haven't had any luck demanding stuff from an AI who was plotting against me. I suppose once they feel they're strong enough to attack me, they feel able to tell me to get lost when I demand things. What does work during a war is demanding things from neutrals, so they can't be bribed into the war or vassal to the AI you're fighting.