First off I need to know which car's interior are we talking about here, then, the condition. Different manufacturers use different materials and they need to be addresses differently. In my experience there is no such thing as a short cut one step treatment. However, what I do is that I remove dust, dirt/grime, luster/oily shine and contamination with a suitable cleaner that can penetrate small pores inside plastic, leather and specially vinyl (rubbery parts like the much touted soft touch padding in the new Corolla). I then use a mild bacteria/odor eliminator on surface (if required), followed by a protectant and top it off with an application of matt finish (in which almost all cars roll out of the factory). Apart from quality products use of various size brushes, pressurized air sprays and fast drying contact cleaners are always on the menu.
