No, he doesn't really have a haunted pen, or at least doesn't admit to it. But there's a good interview with the celebrated author here. I recently read his lesser-known supernatural novel A Stir of Echoes. Recommended. But, unlike Matheson's other early books, ASoE doesn't lend itself to film or TV because it is about a man's internal and highly subjective supernatural experiences.
There's also a good interview with RM in the Dark Dreamers series: