What happened in the interrogation scene in Sicario?

This hasn't really been addressed from the film-makers, but it appears the most obvious interpretation of the scene is that it was a waterboarding.

The original script describes the scene as follows:

A door opens and we find GUILLERMO, 45, fat, three day stubble, and very unhappy to be here. His hands are cuffed behind his back. A DEA AGENT holds a bottle of water that Guillermo sucks like an infant.

Matt walks in.

MATT (CONT’D) Giving him a belly full of water... you devil.

Matt smiles, the agent smiles back.

Matt sits in the only other chair in the room.

MATT (CONT’D) Didn’t think we’d get you here, did you?

GUILLERMO No hablo-

MATT I love it when they don’t hablo.
Brought an old buddy of yours. Bet you ‘hablo’ to him.

The door opens and Alejandro walks in, carrying two five gallon jugs of water. It takes Guillermo a minute, then horror washes over his face.

GUILLERMO El Medellin.

MATT Alejandro, he remembers you.

No expression on Alejandro’s face.

The DEA Agent moves to the door.

DEA AGENT I’m gonna step out.

He closes the door behind him. The red ‘record’ button on the camera mounted in the corner goes out. We notice a DRAIN IN THE FLOOR.

Alejandro walks toward Guillermo, stands across from him. Places a foot on his chest. Pushes... Hard... Guillermo sways back, like a chopped tree. He moves through frame in slow motion, falling out of frame.

From reading it, there's absolutely no implication of rape, but a heavy implication of torture through waterboarding.

The actual scene in the film doesn't quite take place like this:

However, it still appears water-boarding is the most obvious explanation. Whilst we hear muffed grunts during the close-up of the drain at the end, and we notice the unopened water jug, this ignores that for water-boarding to occur the victim will generally have a cloth stuffed into their mouth. This act, along with any other roughhousing Alejandro gives him, would lead to the grunts we hear.

Whilst rape cannot be completely ruled out, it would seem rather out of place. Why would Matt stay and watch? Why would rape lead to more information than torture? If it is rape and torture, again - what would the point of the rape have been?

In conclusion, I can understand where the rape allegations come from, but I think the waterboarding is a far more likely interpretation.

Water cure torture. (Sense of Irony) Victim is forced to drink water until the stomach fills until near bursting and is beaten until the victim vomits and the torture begins again. Dated to around 1898, by U.S. soldiers in the Spanish–American War. Matt's Colleague'Steve' who was filling Guillermo with water ("giving him a belly full of water!, you devil").

The 5 gallon water bottle that Alejandro bought into the room, was to continue the process and maybe even wash the vomit down the drain. Standing so close the Guillermo with his legs kicked apart was a sign of Dominance. Nothing sexual regarding rape.

Waterboarding was not used here due to the lack of table/board with the 20 degree decline. Plus Victims of waterboarding are at extreme risk of sudden death due to the aspiration of vomit.

As I read arguments against this as a rape scene, I see a lot of extra information that is not exactly part of the scene. Villeneuve is a filmmaker that uses all tools at his disposal, and every choice he makes on what to show us matters. Here are the key details presented:

• At the water cooler, Alejandro appears to empathize with another man's family possibly being in danger. He's humanized through the dialogue and Benicio's acting.
• He walks away with a jug of water, and we rightfully assume it's for torture. This is the first bait-and-switch. He's humanized, and in the next second we see how routine torture is to him as he strolls his way to the room.
• After entering the room, Alejandro makes a very strange and unexpected gesture by standing with his crotch in Guillermo's face, exacting sexual dominance. Two guys leave the room, but Matt stays. We know that Matt is ruthless, but due to the look of satisfaction on his face, it might be worse than we think. And what is worse, Alejandro is clearly on board with it, too. So...
• The 2nd bait-and-switch: it happens to both the hostage and to the audience as a result of a brilliant dissonance between the sound and the image. The jug is closed, the floor and drain are dry. And rhythmic grunting begins. The drain is the central focus of this shot for a very important reason. The sounds of torture have begun, but there's no water. That means the form of torture that we expected to happen is not happening: he's being raped instead of waterboarded, and the jug was only used to skew his expectations. Or, perhaps, rape was only the precursor, to be followed by waterboarding or another form of torture later.

Whether it was rape or not, it's strongly suggested to be. The most important evidence in this scene is the details that Villeneuve carefully chooses to show us, and I would argue that few other things matter more when interpreting this. The point of the scene isn't just to show us that they do waterboarding; it's to show us what else they're willing to do. And it's very scary.

Alajandro put his crotch in his face then we hear grunting. Leads me to believe Guillermo was forced to perform oral sex.

The water cure torture or waterboarding is most likely what actually happened.

I think the sexual implications are more visual/audio cues tying the torture to rape, that the torture is akin to sexual assault. You're definitely supposed to think about rape when you're watching the implications of torture.

the water boarding explanation is well thought out and certainly fits with matt's comment and the closeup of the drain and the 5 gallon jug. However, Allejandro's positioning relative to Guillermo is unnatural to say the least! And Alejandro's character is enigmatic and unpredictable throughout the movie. i think the filmmakers wish to underscore these unpredictable aspects of Alejandro's character in this scene in order to make him more interesting. They make it fun for the audience to try and interpret what is happening with the clues given much like in pulp fiction where Ving Rhames character says that he's gonna get "medieval" on the rapists' "ass" with a "pair of pliers and a blowtorch"!