In writing, three is the magic number. Why three? I'll let Algis Budrys explain it:
"Because anything less is unsatisfying, because anything more is redundant, because Aristotle and Lewis Carroll said that what I tell you three times is true. Three times, on a rising scale of effort, commitment, and depth of knowledge of the problem and one’s self, is the correct number."
I noticed this at work in my own writing this morning when I put this paragraph on paper:
"When the next volunteer saw the extraction chair, she stopped. Her eyes widened and muscles clenched as though in stasis. Any moment now, Mark knew, she would do a 180 on those expensive black heels and hightail back to her comfortable life. But she didn't run. Instead, she made the sign of the cross and stepped into the room."
I'm not going to go into how good or bad this paragraph is. What I do want to call attention to is the last sentence. Read it out loud. To me, it feels unfinished and therefore leaves the entire paragraph unfinished. Why? Because the volunteer does two things, not three, which is simply not enough. So I rewrote the sentence: "Instead, she made the sign of the cross, took a calming breath and stepped into the room." Much better.