El elenco está completo, solo que falta el ensayo técnico.

Because completo here describes a state or condition, not an essential characteristic.

• estar + adjective is often used for temporary states or current situations.
• ser + adjective is used more for defining qualities.

So El elenco está completo means the cast is complete right now.

Compare:

• El teatro es grande = The theatre is big.
• El teatro está lleno = The theatre is full.

Here, the idea is that the cast has now been fully assembled, so estar is the natural choice.

Elenco means cast, especially in theatre, film, or television.

In Spain, elenco is understood, but learners should know that reparto is also very common, and often even more common in everyday Spanish from Spain.

So these are both possible:

• El elenco está completo
• El reparto está completo

Both mean that the cast is complete.

Here solo que means something like:

• except that
• it’s just that
• only that
• the only thing is that

So the sentence means the cast is complete, except that / only that the technical rehearsal is still missing.

It connects two ideas:

1. everything is basically in place
2. but there is still one thing left

It has a slightly more natural, conversational feel than a direct pero in some contexts.

Yes, in many situations you could say:

• El elenco está completo, pero falta el ensayo técnico.

This is very natural too.

The difference is mainly nuance:

• pero = a straightforward but
• solo que = except that / the only thing is that

So solo que can sound a little softer or more like there’s just one remaining issue.

Because the subject of the verb is el ensayo técnico, which is singular.

In Spanish, the verb agrees with the subject:

• falta el ensayo técnico = the technical rehearsal is missing
• faltan los ensayos técnicos = the technical rehearsals are missing

Even though in English we might think first about what is lacking, Spanish still treats el ensayo técnico as the grammatical subject, so the verb must be singular.

Faltar often works differently from how English speakers expect.

Here it means:

• to be missing
• to still be needed
• to remain

So falta el ensayo técnico literally works like:

• the technical rehearsal is missing or
• the technical rehearsal is still needed

This is why Spanish uses faltar where English might use:

• we still need
• there’s still... left
• the only thing missing is...

For example:

• Falta una firma. = One signature is still missing.
• Faltan dos días. = Two days remain / There are two days left.

Because with verbs like faltar, quedar, venir, llegar, and similar verbs, Spanish often places the subject after the verb.

So:

• Falta el ensayo técnico sounds very natural.

You could think of it as presenting the missing item:

• What’s missing? The technical rehearsal.

This word order is especially common when introducing new information.

In Spanish, nouns usually need an article or determiner unless there is a specific reason to omit it.

So:

• falta el ensayo técnico = the technical rehearsal is missing

Without the article, falta ensayo técnico, it would sound incomplete or unnatural in standard Spanish.

Spanish generally prefers:

• falta el documento
• falta una firma
• faltan los permisos

Ensayo técnico means technical rehearsal.

It refers to a rehearsal focused on the technical side of a production, such as:

• lighting
• sound
• stage cues
• set changes
• microphones
• timing of effects

In theatre English, people may also say tech rehearsal or tech run.

So this sentence suggests that the cast is already complete, but the production still needs the technical rehearsal.

Because in Spanish, adjectives usually come after the noun.

So:

• ensayo técnico = technical rehearsal
• ensayo general = dress rehearsal / full rehearsal
• prueba técnica = technical test

This is the normal position for most descriptive adjectives in Spanish.

In modern Spanish spelling, solo is normally written without an accent, even when it means only.

So:

• solo que falta el ensayo técnico

is the standard modern spelling.

Older usage, or some writers’ preferences, may still use sólo to avoid ambiguity, but the current general recommendation is to write solo without the accent unless there is a real risk of confusion.

Yes. You could say:

• El elenco ya está completo, solo que falta el ensayo técnico.

Adding ya emphasizes that the cast is now complete.

It suggests progress:

• before, it was not complete
• now, it is

Without ya, the sentence is still completely natural; it is just a bit more neutral.

It is fairly neutral and natural. It could work in:

• everyday conversation
• theatre production discussions
• semi-formal contexts

Nothing in it is especially slangy or especially formal.

If you wanted a more formal version, you might say something like:

• El elenco está completo; únicamente falta el ensayo técnico.

But the original sentence sounds perfectly normal and natural.