There is no real interoperability with the workflow. You export individual clips from FCP, work with them in AE, render the result and replace clip in FCP.
This is, assuming, you aren't trying to import full sequences from FCP to AE which is slightly different. After Effects CS6 can actually import a full FCP7 XML file (listed as an automatic duck file format, due to Adobe's purchase of Automatic Duck, a piece of interoperability software).
Contrary to what the below video states, if you know you'll be working with effects then you should consider working in ProRes HQ. Lower quality footage would be fine for draft sequences/renders but you probably don't want the quality loss for the final product.