Chromakey (Green Screen) Best Practices in Adobe After Effects
Assuming the lighting is solid, you should be in good shape. I haven't used the keying features of iMovie, but I can give you a few tips for After Effects. I'll defer to any power users of AE's keying functionality to chime in with more specific advice.

First, you'll probably want to start with the Keylight plugin (from The Foundry). It should come bundled with After Effects Creative Cloud. There are tons of tutorials, guides, etc. for Keylight on the web.

Second, know that you can create masks with After Effects. Not sure if you're already familiar with them, but masks are custom shapes that allow for specific areas of footage to be visible or modified. Masks allow you to create a "junk matte" around your subject, which can be helpful if lighting gets a little uneven or equipment is visible on the edge of the shot. Masks could also allow you to preserve the actor's eyes so that they are not being keyed (especially if the shot is relatively static). For example, you might want to duplicate the main video layer, mask around the actor's eyes, then pull the key on the duplicated background layer.

Third, if you're still having problems with the hair after firing up Keylight, you'll want to look into the "Despill Bias" option. With despill bias, you'll basically select a skin color to reduce the blue / green bleeding. It might take a little trial and error.