During the Imperial attack on the Rebel cruiser, Princess Leia hides the stolen plans for the Death Star inside R2-D2, in order to keep them from being found by Darth Vader and his stormtroopers.

At this this point in movie, R2-D2 is one of our two main characters. Not only have we been following him around, he's the only character that's been named. So it's interesting how, of the two characters in the shot, he's the one that's most visible. Leia, on the other hand, blends into the smoke almost as if she were a ghost. Considering that she's trying to hide, it's a good choice

In this center framed shot, the main point of emphasis is not any of the characters, but a specific action. Considering that action of giving the Death Start plans to R2-D2 and sending him on his mission is the instigating act that sets the story in motion, it's critical that it's seen by the viewers and its the leading lines that make that happen. The main set of lines are those of the floor walkway, while the three pairs of partition dividers work to create lots of other implied lines. Every line leads to the card she's inserting into the droid.

It looks like there are 3 light sources in this shot. The first is likely a few feet above the camera, representing a ceiling light for the section of corridor we're in. Serving as the main key light, this lamp is illuminating the three sets of partitions, and still has enough reach to light Princess Leia and R2-D2. The ceiling light in the compartment behind our subjects practically only serves to illuminate the smoke acting as a backdrop, but since the smoke is likely only there as part of the damage from the Imperial attack, this light also serves as a reminder of what is going on beyond the scene. The red warning lights provide a splash of color and likewise serve as a reminder of what's happening in the grander scale.

In color theory, red is a color that stands for danger, violence, and aggression, and so its use as the primary color seen in this shot makes sense. The spaceship has just been boarded and our characters are in danger.