articles - see Batman in the theatre

I’m going to see Batman in the theatre.

In the case that I’m going to see a specific movie, e.g. Batman, do I use definite or indefinite article in locative adjunct? (It’s not yet decided which theatre I would go to.)

Answer

As far as I know it's entirely idiomatic, with no particular reason, but when we talk about certain places we commonly use the definite article even when we're discussing an unspecified one out of several places:

I'm going to the theater.

(even if there are several theaters I might be going to)

Bob was in the hospital for three days.

We had fun at the park.

In none of these cases would we substitute "a" for "the".

Also note, British English (sometimes?) uses no article at all for the case of "hospital". ("Bob was in hospital for three days").

As J.R. points out, there are also cases where the indefinite or definite article could both be used:

Q: I can't wait until Guardians of the Galaxy comes out on DVD.

A: I'm not waiting that long; I want to see it in a (the) theater.

Unfortunately I can't tell you what makes this example different from the previous ones.

Edit

Thinking about CarSmack's comment, I think when we use "the" with a place we are implying that we were there to do whatever activity is typical in that place.

I went to the theater [to watch a movie].

Bob was in the hospital [to receive medical care].

If we talk about going to these places to do something non-typical, then using the indefinte article sounds more natural

I went to a hospital to repair the air-conditioning.

or

Q: Where do you work?

A: I work at a hospital [doing IT support].