The methodological break-through after form criticism was Redaktionsgeschichte (redaction history) or redaction criticism. While building on form critical insights, it reaction to the minimalistic view of the evangelists as editors collecting traditions like pearls on a string. Morna Hooker has a memorable line in rebuttal: “It will not, I hope, be regarded as a sexist remark if I suggest that only a man could have used the phrase ‘like pearls on a string’ to suggest a haphazard arrangement of material. Any woman would have spotted the flaw at once in the analogy: pearls need to be carefully selected and graded. And gradually it has dawned on New Testament scholars that this is precisely what the evangelists have done with their material” (The Message of Mark [1983], p. 3). Redaction critics treat the evangelists as authors and theologians in their own right and seek their distinct contribution to the pre-gospel traditions (as largely determined from a form critical analysis).
Though anticipated by W. Wrede or R.H. Lightfoot, it was Gunther Bornkamm (Tradition & Interpretation in Matthew [1948]), Hans Conzelmann (The Theology of St. Luke, [1954]) and Willi Marxsen (Mark the Evangelist [1956]) who really opened the floodgates. However, it may be easier to spot Matthew or Luke’s redactional hand based on how they treat Mark (but see the debate on whether this is the case by James McGrath and Rafael Rodriguez here, here, here, here, here, here, here), but it is much more difficult with Mark because the sources are no longer extant. Premier evangelical scholar Robert Stein’s article “What is Redaktionsgeschicht” JBL 88.1 (1969): 45-56 (Courtesy of Biblical Studies.org) notes what redaction critics look for when they sift Mark’s editorial contribution from pre-Markan sources: seams (remember form criticism assumes the traditions were handed down as independent units which Mark attached together artificially), interpretative comments, summaries, modification of material, selection of material, omission of material, arrangement of material, introduction (Mark’s prologue), conclusion (original ending at 16:1-8), vocabulary and christological titles (p. 53).
Update: For a major critique of the redactional critical method, see C. Clifton Black, The Disciples according to Mark: Markan Redaction in Current Debate (JSNTS 27; Sheffield: JSOT Press; Sheffield Academic Press, 1989).
[...] the form in which we know them today. Michael Patton discussed textual issues in Romans 5:1. Mike Kok introduced redaction criticism. The British Library posted about the New Testament in Codex Sinaiticus. Eis Doxan mentioned [...]