El actor con el papel protagonista da el pie.

In Spanish, singular countable nouns normally need an article or another determiner.

So:

• el actor = the actor
• un actor = an actor

Saying just actor by itself would usually be incomplete in a normal sentence.

Here, con means with, and the whole phrase describes el actor.

So the structure is:

• El actor = the actor
• con el papel protagonista = with the leading role

It works like an attached descriptive phrase: the actor with the leading role.

In more expanded English, you could think of it as:

• the actor who has the leading role

Both can refer to a leading role, but they are slightly different in feel.

• papel principal = principal/main role
• papel protagonista = protagonist role / leading role

Protagonista links the role specifically to the protagonist of the story.
Principal is often a bit more general and is very common too.

So papel protagonista is understandable, though in many contexts learners may hear papel principal more often.

Because protagonista is being used as an adjective-like noun after papel, and its form is the same for masculine and feminine in the singular.

Examples:

• el protagonista
• la protagonista

So:

• el papel protagonista
• la figura protagonista

The word itself does not change to protagonisto or anything like that.

Because da is a normal one-syllable verb form from dar, and monosyllables in Spanish usually do not take a written accent.

So:

• da = he/she gives

Spanish only uses accents on some monosyllables when there is a need to distinguish between two different words, such as:

• de / dé
• tu / tú
• el / él

There is no such contrast here, so it is simply da.

Da is the third-person singular present indicative of dar.

It can mean:

• he gives
• she gives
• it gives

Spanish often uses the present tense:

• for habits
• for summaries
• for stage/action descriptions
• for telling what happens in a scene

So this sentence may describe what happens in a play or in a general situation: the actor with the leading role gives the cue.

Dar el pie is an idiomatic expression. It does not literally mean physically giving someone a foot.

It is used especially in theatrical or conversational contexts to mean something like:

• to give someone their cue
• to prompt the next line or action
• to provide the opening for someone else to speak or act

So even though the words are literally give the foot, the actual meaning is idiomatic.

Because the expression is fixed as dar el pie.

Many Spanish idioms include an article that English would not necessarily use. Here, el pie is part of the set phrase, so you normally keep the article.

Compare:

• dar el pie
• dar la cara
• tener razón

These are learned best as whole expressions, not word by word.

It is understandable, but depending on context, Spanish speakers might also say more natural-sounding alternatives such as:

• El actor protagonista da el pie.
• El actor principal da el pie.
• El actor que tiene el papel protagonista da el pie.

The original sentence is grammatically clear, but it is a bit more descriptive and slightly less compact than el actor protagonista.

Not normally. In this context, con is understood as describing a characteristic: the actor with the leading role, meaning the actor who has / plays that role.

Spanish uses con in this kind of descriptive way quite often:

• la chica con gafas = the girl with glasses
• el hombre con barba = the man with a beard
• el actor con el papel protagonista = the actor with the leading role

So it identifies which actor we mean.

Usually, no, not in this sentence.

Con el papel protagonista points to a specific role: the leading role.
If you remove the article, the phrase sounds much less natural here.

Spanish often uses the definite article where English learners may expect no article:

• con el coche rojo
• con la idea correcta
• con el papel protagonista

So con el papel protagonista is the expected form.

Because el pie is the direct object of dar, but it is not a person.

The personal a is mainly used before specific human direct objects:

• Veo al actor.
• Llamo a María.

But here:

• da el pie

There is no personal a because el pie is part of an idiomatic object phrase, not a person.

In standard Spanish, if the person is female, actriz is the usual word.

So:

• el actor = male actor
• la actriz = actress / female actor

If you were specifically talking about a woman, you would normally say:

• La actriz con el papel protagonista da el pie.

A native speaker would usually match the noun to the person’s gender in this kind of sentence.