In brief, we are inclined to go along with someone’s suggestion if we think that person is a credible expert (authority), if we regard him or her as a trusted friend (liking), if we feel we owe them one (reciprocity), or if doing so will be consistent with our beliefs or prior commitments (consistency). We are also inclined to make choices that we think are popular (consensus [social proof]), and that will net us a scarce commodity (scarcity).
In marketing automation, Ryan Deiss, co-founder of Digital Marketer, often describes the sales funnel as a multi-step, multi-modality process that moves prospective browsers into buyers. It's multi-stepped because lots must occur between the time that a prospect is aware enough to enter your funnel, to the time when they take action and successfully complete a purchase.