El elenco puede sobreactuar y salirse del personaje.

Because el elenco is a singular collective noun. It refers to a group of people, but grammatically it is singular, so the verb also stays singular:

• El elenco puede... = The cast can...

This works like English the cast is/can.
If you named the individual actors instead, you would use the plural:

• Los actores pueden sobreactuar...

Yes. El elenco means the cast: the group of actors in a film, play, or show.

A useful note for Spain: el reparto is also very common for cast, and in many contexts it may sound even more natural. But el elenco is perfectly correct and widely understood.

Examples:

• El elenco de la película = the cast of the film
• Un elenco excelente = an excellent cast

Sobreactuar means to overact.

It is built from:

• sobre- = over-
• actuar = to act

So it works very much like English overact.

Examples:

• Ese actor sobreactúa mucho. = That actor overacts a lot.
• No sobreactúes. = Don’t overact.

Because they come after puede.

After modal verbs like poder, Spanish normally uses the infinitive:

• puede sobreactuar
• puede salirse

This is similar to English:

• can overact
• can come out of character / can break character

You do not conjugate the second verb after poder:

• correct: puede sobreactuar
• not correct: puede sobreactúa

It means to come out of character, to stop being convincing as the character, or to break character.

Literally:

• salirse de = to come out of / to leave
• el personaje = the character

So the idea is that the actor stops fully staying inside the role.

In performance contexts, this can mean:

• the acting no longer feels believable
• the actor momentarily stops behaving like the character
• the performance becomes artificial or inconsistent

Because salirse de algo is a common pronominal structure meaning to come out of something, to leave something, or sometimes to go outside the limits of something.

Here, salirse del personaje means more than just physically going out; it means slipping out of the role/character.

Compare:

• salir = to go out / leave
• salirse de = to come out of / to step out of / to slip out of

So the se is part of the expression here.

Because de + el contracts to del in Spanish.

So:

• de + el personaje → del personaje

This contraction is normally mandatory.

A similar one is:

• a + el → al

Examples:

• Viene del teatro. = He/She comes from the theatre.
• Va al cine. = He/She goes to the cinema.

Usually can is the best translation here.

Poder can express:

• ability = can
• possibility = may / might / can

In this sentence, puede suggests it can happen or the cast can end up doing this, so in English can is the most natural choice.

Depending on context, it could also feel close to:

• may overact and break character
• can overact and come out of character

Because one modal verb can govern two infinitives joined by y:

• puede sobreactuar y salirse del personaje

This means:

• can overact and come out of character

Spanish does not need to repeat puede:

• puede sobreactuar y salirse... = natural
• puede sobreactuar y puede salirse... = possible, but more repetitive and usually unnecessary

In this sentence, del personaje most naturally means the character they are playing in a general sense. Spanish often uses the singular article this way.

So even though el elenco is a group, the sentence is talking about the idea that cast members can fail to stay inside their roles. Spanish does not need to spell that out every time.

If you wanted to be more explicit, you could also say:

• salirse de su personaje = to come out of their character
• salirse de sus personajes = to come out of their characters

But the original sentence sounds natural as it is.

It is a very natural expression, especially when talking about acting or performance. It is not the only possible wording, but it is a common and understandable way to say break character or come out of character.

Other possible expressions include:

• romper el personaje
• salirse de su papel
• dejar de sonar creíble como personaje depending on the context

Still, salirse del personaje is clear and idiomatic.

Spanish often keeps things compact. A more expanded version could be:

• Los actores del elenco pueden sobreactuar y salirse del personaje.

That makes the people involved more explicit: the actors in the cast.

But the original:

• El elenco puede sobreactuar y salirse del personaje is perfectly grammatical and natural, with el elenco treated as one unit.