Redaction criticism regards the author of the text as editor (redactor) of the source materials.
Instead, it focuses on how the redactor shaped and moulded the narrative to express theological and ideological goals.
There are several ways in which redaction critics detect editorial activity like the following: Although redaction criticism (the possibility of the various gospels having different theological perspectives) has existed since Antiquity, three modern day scholars are regularly credited with this school's modern development: Gunther Bornkamm, Willi Marxsen and Hans Conzelmann[1] (see Bornkamm, Barth and Held, Tradition and Interpretation in Matthew, Marxsen, Mark the Evangelist, Conzelmann, Theology of St Luke).
If a writer consistently avoids reporting, for example, the weaknesses of the Twelve Apostles, even when there are earlier sources that provide lurid details of their follies, one could draw the conclusion that the later editor/author held the Twelve in higher esteem because the editor had presuppositions, or because the editor was perhaps trying to reinforce the legitimacy of those chosen by Jesus to carry on his work.
By tracking the overall impact of this editorial activity, one can come away with a fairly strong picture of the purpose of a particular text.