Difference in serialization of .NET ScriptService when using POST vs GET?

Question

I am calling a method in a [ScripService] from the client side (JS). When I use POST to make the call, everything works fine. But when I use GET this error is thrown:

There was an error reflecting type ... Cannot serialize member xxx of type System.Collections.Generic.IList`1[[System.String, mscorlib, Version=4.0.0.0, Culture=neutral] because it is an interface.

I used fiddler to send the same POST request, which works. Then I edited the request to use GET (in fiddler), everything else is the same; I get the same error as before.

Why does this happen? At the moment of execution, the member xxx is of a concrete type, and it serializes OK when the request is POST. What am I missing?