Is a movie played in a theater or at a theater?

In the US, the phrase is "at the theater", and it's usually more specific than that -- "Die Hard 26 is playing at the Googleplex theater." The phrase "in a theater" would only be used if there was a question as to whether it was being played in the usual situation or outside, at someone's house, or in some other location.

In the US, the verb play is usually active with regard to movies, rather than passive.

• The movie is playing at the Bijou.
• The movie plays in local theaters before going to DVD.

Both prepositional forms, at and in are regularly found, but in is more common with the generic term theater and at with the specific theater name.

With the verb show, the passive form is found, and in is more common.

The movie is being shown in a theater.

With truncated forms, either preposition can be used:

• Now in a theater near you!
• Now at a theater near you!

"at" is normally used if you're stating a specific place like "at the top of the closet", or "at the Central Plaza" and "in' is used if you're not stating a specific place like "we'll meet in China next month". it is specifically stated that the place will be in China but not where exactly in China. ;) (if I stated something wrong, please let me know)

"In" is used when talking about a closed space where someone physically is. For example: 'I am in the theater' means that I am physically in the building. 'At' is used more as a general meaning. 'I am at the theater' would refer more to the concept, like I went to see a movie, and not that I am in that exact location.

Other examples:

• I am in the office. (in the physical space)
• I am at the office. (I am working, I am somewhere in my workplace)

Exception of this are geographical names, we always use "in" with countries, cities, etc.

But even here, sometimes we see this same rule:
I am at the lake (somewhere close to the lake, the exact location is not important) or
I am in the lake (I am physically in the water).