It's hard to be happy when you are facing 120 to 140 degree temperatures and nothing seems to be moving in a direction that you think or they think or you've been told it's supposed to be moving in.
I don't know if it's harder but when you're playing a real person you want to honor their memory - even if they're a criminal or someone that the public loathed. That can be challenging.
I have always wanted a solo career, deep in the darkest pit of myself, but I didn't dare admit it to myself even. It took me a long time to confront my fears.