you're right, a marker interface doesn't provide methods to implement. It really just tells you something about a class, e.g. if a class provides random access or is serializable. There's nothing mysterious about marker interfaces. With current Java versions you could better use annotations for this purpose.
This will allow you to save the creating of the Iterator if the List also implements RandomAccess. I've never done this though; IMHO, an Iterator has little extra overhead in both memory and speed and therefore not worth the trouble. In fact, the ArrayList's Iterator implementation (actually implemented by AbstractList) uses get(int) in the background as well. It's LinkedList that has a special implementation that uses the special structure of the list.