Is this sentence from a movie grammatical?
I thought you overqualified.
The construction "think someone adj." strikes me as odd. Explanations/examples?
It is indeed an accepted construction. It sounds a little more eloquent that the regular "thought that you/she/they was/were + adj.", so it will often be used in fine literature (or when the speaker is trying to emulate this "higher" style).
Examples of usage:
— Do you think the earl really thought him suitable for someone's husband? (source)
— Because of the way Robert spoke and carried himself, Matthew thought him educated. (source)
— He was a decent, hardworking man and she thought him well-adjusted. (source)
It is similar to the construction where the verb is "to find". The following phrases will probably sound more familiar to you:
to find somebody attractive/arrogant/ugly/well-mannered, etc., etc.
So it is used to express a certain opinion about a person.