Environment should always play an important role when building a scene, but it is particularly crucial when it comes to setting the pace and mood of each scene. The way you structure the environment and how the characters respond to it can indicate a characters' unspoken emotions can and even hint at circumstances to come. When building an environment, I think many writers struggle with establishing what to include in the details. This struggle may stem from the fact that most everything can be used to enhance the atmosphere of a story. Merely describing a warm, sandy beach allows the reader to picture the place. Showing the effects the weather has can make that picture even clearer. But having your characters react to that environment and describing intricate details about the space can really bring the environment to life. So here are some things you can focus your attention on.
Lighting. It's easy to establish whether it's day or night during a scene. However, writers often forget some of the other effects that time of day can bring. For instance, if it's night, is there a full moon, a partial moon, or no moon visible at all? Different moon phases affect the light reflected off it, thereby altering shadows as well. Certainty of what one can see in the surrounding area depends strongly on lighting, and characters' (as well as readers') speculations about it can change when an environment is dark or dimly lit. Keep in mind how the lighting of any given scene might affect surrounding objects and characters, and use it to your advantage; describe physical features of characters and/or objects using these changes.
Time of day/Day of the week. Time isn't something writers always focus on in great detail for scenes, but it does have a direct effect on the environment and can help establish one. During rush hour on a weekday, for example, there would be much more traffic (and background noise) on a main street than there would be on the weekend. On the other hand, a place like a mall might experience more crowds on the weekend rather than a weekday, especially if it's a holiday. Take time of day and day of the week into consideration when it comes to the surroundings during a particular scene. You might be surprised how it can change the outcome of your characters' actions or responses to one another. It might even introduce unforeseen conflict.
Reaction of the characters. Believe it or not, even dialogue and body language can help build the environment and pace of a scene. How you respond to the things around you can indicate quite a bit about what is going on. Shivering for instance, indicates the temperature might be chilly. Fidgeting and darting eyes show nervousness, a clear sign that something bad has happened or is about to happen. And short, choppy dialogue can indicate action, conflict, and emergency situations, adding to the pace already established by body language. If you're not sure what body language to use to convey specific emotions, check out the book The Emotion Thesaurus by Angela Ackerman and Becca Puglisi. It gives a wide range of actions that characters can perform to express emotions without dialogue.
Senses. In any given environment, there will be smells, sounds, textures, tastes, and a sense of temperature. You don't need to describe every one of these for every scene you write, but providing as much detail as you can, particularly for new or unfamiliar objects/places, can really enhance the scene. An old building holds far different features than a new one, and a busy city has a far different kind of background noise than your typical suburb does.
Nature. For the most part, writing in a few descriptions about the weather is easy. It's one of the first things a writer learns to incorporate into his/her writing as part of the environment. What is easy to forget is the rest of nature, especially the living parts. If you're writing a story that takes place in an existing geographic location, research the plant and animal life in the area. Find out what types of plants grow there and during what time of year. Find out what animals are natural inhabitants of the land (and water). One of the biggest complaints I hear from people when a book is written about a city they are familiar with is that the writer got the environment wrong. They mention animals, plants, and sounds that aren't really prominent there, or they get the overall sense of culture wrong. If you're picking a place you're unfamiliar with, make sure you know what you're talking before tackling the project of its environment.
Foreign objects or material. New and unusual objects are sure things to take note of. The technique of describing an object in detail is also effective for noting red herrings giving hints about later plot points. If the object is something a character comes in contact with, describe the texture and temperature of the object as well as its shape, size, and color.
My overall advice about establishing environment and pace is to include as much detail as possible in your first draft. It's much easier to remove things later than it is to add them in.