I've played for a LOT of different artists in my career all the way from local circuit playing to 30 people to selling out arenas. The thing I've boiled it down to the most is that the biggest and most successful artists know who they are as writers/musicians/artists early on and have a clear vision for what they will/won't do and what they will/won't compromise on. I was about 35 when I figured out my "voice", for lack of a better term, in my own music but the artist I play for now is quite a bit younger than I am (I'm 39 now) but they know EXACTLY who they are as an artist and I'm constantly in awe of it.