When Actors Struggle To Master A Character

Scene Is Boring
Some scenes are more subtle than others and that can be taken as boring to certain actors. Your character may not be doing much physically or even verbally, but it doesn't mean nothing is going on. The scene may not be written perfectly or could use a rewrite, but there is a lot you can do to make it interesting for you as is. What is going through the character's head that he or she is not saying? Are they contemplating actions that they don't take? Are there things they want to say but never get up the nerve? Though the script may not call for it, there are lots of things you can do to express your thoughts about the character using stage business and non-verbal noises.
Don't Like The Character
When you hate someone it's hard to do things they want to do. Just like in life, when we don't like someone we don't want to be around them. The character may remind you of someone you hated in your life or may just come across as a mean person. Instead of quitting, give the character really good reasons for being who they are. Don't let them just be an asshole, make them an asshole who was once really nice but was consistently mistreated by close friends and relatives. Even your mortal enemy has a good reason, find one for your character.
Can't Empathize
You may not hate the character, but you may not find what they want particularly compelling. You've never been ambitious so you have trouble playing a character like that. If you can't find common ground with the character, you will have trouble playing them accurately. Let go of some of your worldview and fully explore this character's world. Use "what if" questions to put yourself in a situation that is similar to something you do care about and see if it doesn't help.
No Life Experience To Substitute
This is related to all the other reasons. If you know little about life, you will be easily bored. With a strict idea of what people should be like and little capacity for empathy you will find most people alien. There is not much to do with this problem except go out and do something besides acting for a long time. Actors are trying to bring life to fictional stories, if you don't know much about life you will be incapable. You can probably get by with faking it for a while, but sooner or later you will need a lot of life experience to fuel your art to a level of greatness.
When you find yourself stuck with a role, slow down and look at what your mind is telling you. There are usually clear reasons why you can't get into the character and there are solutions for them. Don't get frustrated that you don't come to it easily or intuitively, learn from the experience and get better as a result of it.

