Okay, I believe that visualization is the key to all creativity. To do it properly, I try to shut out all distractions and go inside my head to listen. The pictures may not come at first. I seek out a place where it is quiet; where I feel comfortable. I have such a place on my back porch behind several potted plants. The dogs and cats generally join me there. The energy is perfect. It is in this place that I'm most likely to hear the muses speak. I never draw while I'm there. In fact, I like my conscious mind to shift into neutral; it's a safe moment reserved for the romp of unbridled thoughts. If at first you don't suddenly begin seeing fantastic visions, it's okay. Be patient! The good stuff will come. I guess it's a lot like meditating. Here is a word of advice: Keep your mobile phone far away.
Well, the best source of inspiration is your very own life experience. Actors have an exercise that helps them draw upon that resource titled "emotional recall." Some parts may be outside the actor's own life experience in which case, he/she must do interviews and lots of research. It is always a challenge to find the character. I lament that many animators don't do this exercise thoroughly enough and resort to mugging; drawing funny faces. Pulling faces is definitely not acting!
A subject that all animators know about, just remember do drink a lot of water. Coffee tends to dehydrate you and if you don't drink enough fluid it can leave you a little bit worse for wear.