Untrained actors act the adjectives and adverbs; trained actors act the verbs.
“Acting the verbs” means figuring out what your character wants and trying to get it. This is really pretty simple, although it requires a large shift in how you look at a scene.
Instead of thinking of your character as being “needy”, think in terms of “I want you to love me.” Instead of thinking of your character as being “bossy”, think in terms of “I want to make sure this party comes off perfectly, because I will feel embarrassed if it isn’t successful” or “because I need to control my world in order to feel safe” or “because if it is successful, I will get the job I want more than anything in the world.”
I hope you can see that the three options I’ve given for why your character might be bossy are very different, and will probably produce different results. The sort of “bossy” you are will change, because what is driving it is different. But if you just play “bossy”, you’re apt to go for the same kind of “bossy” no matter who your character is or why he is doing what he does.
And let’s be honest: in your real life, do you ever decide to “be” bossy? No, you decide you want to give the best party ever. You decide that the people around you are incompetent or slow or uncreative, and they need you to direct them. You have a strong attachment to your vision, and you want to see it achieved. You may end up bossing people around and irritating them in the process, but it’s not like that’s what you’re striving to do. You’re just trying to give a really good party.
EVERYTHING we do in our real lives is in service of getting something we want: a quart of milk, an answer to a question, an experience, a job, love, pleasure, prestige, power, money. No matter how trivial the goal is (I want to read a book I am enjoying; I want to snack on something to take off the edge before dinner; I want to straighten that picture on the wall because it’s driving me nuts), EVERYTHING we do is to get something we want.
It’s the same for your character. EVERYTHING your character says or does is in service of getting something she wants. It isn’t about feeling her emotions. The emotions are simply a by-product of who you are, what you want, how you try to get it, and whether or not you are successful in getting it.
See Part I here. See Part III here. See Why Playing the Emotions Doesn’t Work here. See Why Playing the Verbs is (Ultimately) Easier than Acting Emotions here. See Choosing Verbs here. See Big Verbs vs. Little Verbs here. See How to Learn to Play the Verbs here. See An Example of Why Verbs Make a Difference here.