Been feeding live for 10 years and have only had one incident where a rat seemed aware of what was going to happen and was aggressive at defending itself. Snake is fine and the couple of small scratches shed out, I intervened before there was a real fight. No incidences with my ball python thus far and have had him for a year. He was 3 yrs old when I got him and the previous owner was feeding him mice. Second time I fed him, he was moved to small rats. First small rat I put in there he gobbled right up and is now up to medium rats with no problems.
Most times I observe the rat to be either curious or scared and cower in the corner. So the animals are either nose-to-nose before the snake strikes or he waits for the rat to pass his hide. Petey is a very efficient or lazy hunter. My rat snake actively hunts but sometimes stays in his hide and waits. My king and milk snakes actively begin hunting once the rat is dropped in.
Had a few instances where the animals seem to "make friends". They'll sleep in the hide together and leave each other to their own agenda. Either the rat will be gone when I get home from work the next day or I take it out and throw it in the holding tank until the following week.
Sama and AKA Reptiles have good advice. Be observant and start on rat fuzzies. Knowing your snake's habits and how it acts around its prey will help. Eventually you will know the nuances of how rats act as well. I have confidence in my snakes' abilities to hunt prey items(although my milk snake needs a major lesson in constricting) so I have little to no worries when I throw rats in with them.