As Augustine says, the statements and precepts of sacred Scripture can be interpreted and understood from the actions of the saints, since the same Holy Spirit who inspired the prophets and the other sacred authors is the Spirit who drives the actions of the saints. As we read, Moved by the Holy Spirit holy men of God spoke (2 Pet. 1.21); and For all who are led by the Spirit of God are sons of God (Rom 8.14). Thus, sacred Scripture should be understood according to the way Christ and the other saints observed it in their practice.
(Quoted in Gilles Emery, The Trinitarian Theology of St. Thomas Aquinas, trans. Francesca Aran Murphy, 261)
In a simple and clear way, I think this passage explains what we mean as Catholics by 'Scripture and Tradition', and especially what Dei Verbum is getting at when it says that these form "a single deposit of the Word of God" whose life in the world is a mutual interconnection and intercommunication. (9-10)
This is why the Bible can only be properly interpreted from within the Church; to try to do so outside of her--as many do--is to divide God's single act of self-revelation and thus to arrive at conclusions that are incomplete and impoverished. Tradition interprets Scripture. This is not to say that 'traditional interpretations' are normative, in the sense of tradition as a species of human conservatism, but to say that the lives of the saints in the broadest sense of the revelation of the Holy Spirit in the concrete, historical sanctification of the baptized down through the ages is the interpretive key to the Scriptures which in turn are normative for the Church.
For us Franciscans, this helps us to understand that the statements of Francis at the beginning of the Rule and at the end of the Testament go together in a fruitful dialectic. "The rule and life of the Friars Minor is this: to observe the holy gospel of our Lord Jesus Christ," which we then strive "to observe in a more catholic manner."