reveal a reel
The Spanish phrase 'revelar un carrete' translates to 'reveal a reel' in English. It's often used in the context of developing film from a film roll, a process which brings the captured images to life. In a broader context, it may also suggest the notion of unveiling or uncovering something.
I am going to develop a roll of photographs that I took on the trip.
This sentence is indicating that the speaker is planning to develop (or 'revelar' in Spanish) a roll of film ('un carrete') which has photos from a trip they took.
Pedro needs to develop a roll before showing us his latest portraits.
In this example, Pedro has to develop a roll of film, presumably containing his recent portrait photos, before he can present them to the ones the sentence is referring to.
By mistake, Mariana went to develop a roll that had already been developed.
This sentence describes a situation where Mariana mistakenly tries to develop a roll of film that has already been developed, using the phrase 'revelar un carrete' to explain the action she mistakenly carried out.