First, the writing is sparse. There's no room for flowery descriptions to breaking down a setting into minute details. There are production designers and wardrobe supervisors that will decide how a particular scene is going to look. Your job as a writer is to set the tone in as few words as possible; their job is to interpret that tone and create a scene with the appropriate detail. For example, an action element in a screenplay describing a room is likely to say something along the lines of, The living room looks like a carbon copy of a Crate & Barrel ad, which establishes a clear visual without going into detail. The production people would be the ones that decide your description means a white-leather mid-century modern sofa with an espresso-colored oak coffee table set against ecru wallpaper.

Second, although I will in all likelihood tell you that you can bend or even break this rule later, let's start with the general concept that the information that goes into an action element should only be what an audience can see or hear. Remember that your audience in all likelihood won't have the opportunity to read your screenplay in advance of watching the movie, so if you write something into an action element, make sure it's something that can come across on the screen or through the speakers in the theater. Descriptions like, She thinks about what to make for breakfast are meaningless to an audience member. A physical representation of that thought process like, She stands in front of the open refrigerator door, staring at her options gives the audience a way to understand and process the information you're trying to get across.

Third, you don't have to write in complex sentences or even complete ones. It's all about driving the visuals and the story forward on the page. Unlike with publishing where the audience will see your words on the page because the text is the finished product, screenwriting is meant to be a template that professionals will use to create their finished product (the motion picture). Because the script is a template or working document, it's become acceptable to take certain creative liberties in the way you write your action elements. You don't have to write, With his gun out, he checks the living room for the suspect, and then moves into the kitchen. He finishes his sweep of the house by checking the master bedroom and comes up empty. It's perfectly acceptable to write, Gun out, he sweeps the house. Living room and kitchen... empty. Master bedroom. Empty. Screenwriting is about presenting information in a dynamic and abbreviated way more than it's about impressing the reader with your sophisticated writing ability.

Fourth, another good rule of thumb (which I'll probably also tell you is okay to bend or break later) is to operate under the assumption that a new paragraph is a different focus of a scene. Each paragraph (which is its own element) should be about one focal point in a scene, and when you change the paragraph, you change the focus. Just imagine that you're watching the movie in your head, and each time the camera cuts to something else, you change the action element. And while the advice in "Elements of a Screenplay"  was that you generally shouldn't exceed four or five lines of description per element, there's absolutely no minimum. Play around with that space for your benefit. Here's an example of a scene from one of my screenplays:

Talia and Claire enter a large parking level filled with rows of vehicles. It’s eerily quiet.

Claire motions for Talia to take one row while she moves down another. Both unholster sidearms and hold them at the ready.

They creep slowly and quietly down the row, checking each vehicle as they pass. They’re almost finished with the first two rows when FOOTSTEPS echo through the garage.

Claire and Talia both spin around, weapons trained, fingers quivering on their triggers.

It’s Scourge. He puts his hands up in mock surrender.

The women glance at one another across the row of cars, taking a deep breath.

As you can see, each element has a different focus. The first one is the two characters (agents) arriving on scene. The second element is one of the agents signaling to the other which, although it could be part of the same shot, is a second or different shot in my mind than the one that brings them in. Then the third shot is them slowly checking the vehicles when they hear a sound. The fourth is them whirling around at the sound, the fifth is the shot revealing the sound came from their partner, and the sixth is the reaction shot of the first two agents when they realize it's not the person they're after. On the page, the length of each of those elements is 2 lines, 2 lines, 3 lines, 2 lines, 1 line, 2 lines... and the length of the action element and what's included - when strung together - is what creates a sense of pace. If you want to really develop the setting and slow things down so you're focused on nuance, longer action elements slow the pace. If you want something to be a quick, staccato series of shots to create a rushed, frenetic pace, shorter action elements quicken the pace when strung together.