Acting inside 2D world

It sounds like you have the right general idea, yes. 
- Shoot actor on greenscreen
- Key out green background
- Add animated background
- Animate elements around your actor

Of course, the big step I left off the beginning is PLANNING. If you're just starting out at this, look up a few videos about shooting on a greenscreen and working with greenscreen footage. Spend some time getting comfortable with the technical requirements and make sure those are hammered out. When you feel like you're not going to be battling the equipment, then you'll want to carefully plan out a scene - what your actor needs to do & how they might interact with objects that won't exist until later. Using references with real objects (that you can later remove with masking or other compositing techniques) is helpful. 

Doing this well takes planning, and you'll get much better at it with experience. Don't be afraid to experiment and have fun with it, and be realistic with yourself at first - it'll likely take some practice before everything looks the way you imagine it. 🙂 

Yeah, it's mostly preparation and figuring out where everything is in the scene. In addition to just having a green screen you'd simply have all sorts of marks in the scene like tape on the floor or creatively using e.g. light fixtures as references or the old "tennis ball on a stick" plus if you have situations where people need to sit on something or step over an obstacle you have to have a cardboard box or similar in the scene also painted green. In any case, the most important aspect of this is measuring out everything. You have to know how far the camera is away from the scene and you have to know how objects are placed or else even the best acting can't save you.

Mylenium