Some Gadites defected to David at his stronghold in the wilderness. They were brave warriors, ready for battle and able to handle the shield and spear. Their faces were the faces of lions, and they were as swift as gazelles in the mountains (1 Chronicles 12:8).
The Bible clearly says that the faces of these Gadites "were the faces of lions." What is more, we are told that they were "as swift as gazelles in the mountains." That's pretty fast. They must have been mutants, right? No.
What do we have here? Anyone that has learned anything about literature would understand that writers throughout history and in every nation used figurative language. In English literature, they have even been named. For example, "their faces were the faces of lions" would be considered metaphor. Google defines it as
A figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable.
So the expression compares the faces of the Gadite men to that of lions; thus, expressing fierceness, fearlessness, and terror for those they go against.
Again, the expression "they were as swift as gazelles in the mountains" would be an example of another figure of speech called simile, which is defined as "a comparison of one thing with a another thing of a different kind" using the words "like" or "as". Instead of simply saying these Gadite warriors were fast, the comparision with gazelles would stir the imagination and focus the reader's attention on their swiftness.
If we do not recognize the use of figurative language in the Bible, then we will produce some very unusual and sometimes nonsensical interpretations. For example, consider this literal rendering of passages from the Song of Solomon (e.g. Song of Solomon 4:1-5), where Solomon praises the beauty of his bride (see picture):
So, we must recognize that the Bible uses figurative language to convey truth. Metaphor, simile, hyperbole, metonomy, etc. You can find a lot of these described on the internet with examples from the Bible. Just search for something like "Figures of Speech in the Bible". One of the greatest book ever published, which seeks to provide a comprehensive discussion of every possible figure of speech used in the Bible, is by E.W. Bullinger from the 19th century. You can purchase his book at Amazon.com, or you can download a free PDF version of it here: Holy Bible Institute.
Now, recognizing figurative language is used in the Bible does not mean that the Bible should not be interpreted literally. As one astute commentator wrote, every figure of speech has an underlying literal truth. And most of the Bible is not written in figurative language and much of it is simply history or expository teaching. Therefore, the tried and true method of interpretation, which involves a contextual, literal, grammatical, and historical interpretation of the text still stands true.