The LA Times has posted another article and they have interviewed  James and Oliver Phelps!

MG: What was your favorite experience working on “Half-Blood Prince?”

OP: That would be being a part of the Fred and George joke shop scene. The crew built an exquisite set and there are a lot of special effects involved to simulate the magical pops and explosions. It’s a great scene.
JP: I really enjoyed working with the crew. Most of my actual acting I was able to complete in about two weeks. When I wasn’t acting I would help the crew set up scenes and shots. I learned a lot more about direction and filmmaking in general and that was pretty cool.