What's "break out" mean and when's it used?

Is it okay if I just say "I'm excited" instead of "It's exciting for me"?

Yes, there isn't a huge difference between the two, as both are expressing the actor's excitement at playing this character. Ex: It's exciting for me to go on vacation. Ex: I'm excited to go on vacation.

What does "firearm" mean?

A "firearm" refers to a portable gun like a rifle, pistol, or something that can fire bullets. They mention all these weapons here since they're known to be the Avengers and to have some kind of weapons on them in the movies. Ex: It's illegal to carry firearms in Korea as a civilian. Ex: I've never shot a firearm in my life.

What is the meaning of "after all"?

"After all" is used to confirm something in spite of any problems or doubts. It is similar to "above all" or "ultimately". Ex: We are siblings after all, so we're destined to get into fights. Ex: You are a good student after all, so you shouldn't worry about one bad grade.

What does "read [someone]" mean?

To "read [someone]" means to understand someone's thoughts and motives, to have a sense of what they're feeling or trying to say. It's also possible to "read" a situation and understand the dynamics of something happening without it being clearly said. Ex: I always struggled to read her when we had a conversation. Ex: He reads me so well and always knows what to say. Ex: I read the situation entirely wrong. I thought you were hitting on me.

What does "just" mean here?

"Just" is an adverb in this case an means "only". "Just" here is a part of the expression "just in case", which is a common expression that is used to express an action of doing something based on the possibility of something else happening. Ex: Let's bring an umbrella just in case it rains. Ex: I packed extra shirts just in case.