What topics/areas of algebraic geometry (aside from perverse sheaves/D-modules, etale cohomology, and possibly derived algebraic geometry) is it useful to learn/master if one is interested in doing research in geometric representation theory? What are the best sources to learn these topics? How does one know that one understands the material on a deep enough level to do interesting research?
My impression is that reading almost all of Hartshorne and solving a large portion of problems there (+ reading some specialized literature in geometric representation theory) is insufficient to solve interesting research problems. How different is preparation for research in geometric representation theory from preparation for research in other areas of algebraic geometry? Does one need a deep understanding of differential geometry and analysis and how to gain it, or are there specific areas/topics in geometry and analysis which are more useful to learn/understand?