A, B and C and also what eagl said. Although I had D happen to me many times yesterday.
There's nothing I hate more than working a bandit for 2 or 3 min taking an advantage away from him, only to have somebody come hit his stalled plane 2 seconds before I can get around to kill him myself. This happens alot too. Remember, just because the bogie's on my 6 doesn't meen he has the advantage

I think it's always better to stay about 1500 ft. or so above the fight (if your bud has the advantage) and protect him. If the fight is neutral, I will usualy make a NON-shooting pass to distract the bogie and hopefuly give my friend the advantage. Otherwise I stay high and wait for more bogies or for my friend to lose the advantage. Just by being there though, you can cause the bandit to turn with you in mind which can give your wingman the break he needs.
Then ofcourse there are the times when I NEEEEEED that kill and I just go in guns-a-blazing....
Udie