Dialogue is like music. Words, sentences, even paragraphs have a rhythm to them. Change the rhythm and you affect the pace off you story. For example, if you want to convey a hurried movement of action, your dialogue might consist of short words, delivered quickly. You could also rely on a few breathless sentences, with no extended moments of silence between them. To slow things down, add thoughtful pauses, punctuation, a more languid delivery or choice of words. Here are some examples:
"No. He just wants the files. I need to get over there so if you know where they are, call my secretary and she'll bring them over. Okay?"
"No... he just wants the files. I need to get over there, sometime. If you know where they are, call my secretary, will you? She can bring them over."
The two sentences are the exact same length, but the feel of them is completely different.