The -er form of an adjective (fast, faster, fastest) is the comparative, comparing one noun to another: she is faster than I am. When comparing verbs, however, an adverb must be used. Commonly, the positive (the non-comparative, non-superlative; fast in the above example) adverb is the -ly form (quick = adjective; quickly = adverb; fast, in the above example, is both adjective and adverb). The comparative form of the -ly adverb is formed by adding the word more: quickly, more quickly. If nothing else, however, because of efficiency, often the comparative adjective is used in place of the comparative adverb, as in the Podcast quoted here. But if a verb is being modified (left, in the Podcast), the adverb, comparative or otherwise, is required.