One of the most important items of shooting these types of Green screen, Virtual set shots, or plate shots that are overlooked by beginners is the camera placement. Look closely at the virtual set, or the plate shot you are using. WHERE is the VIRTUAL CAMERA sitting. What angle, what distance, what kind of lens is the VIRTUAL CAMERA using? Determine these as best as possible then MATCH YOUR CAMERA shot to the virtual set, or plate shot camera set up. If you shoot your subject from an incorrect angle to the virtual set or plate it will never "feel" correct. Once you have determined the correct data and you are successfully matching the plate or vittyual set.. Try these ideas to help embed your subject in the room.
look closely at the lighting and colors in the room. If you have the virtual set prior to shooting, then rig your lights and include a bit of the same "color" for your subject that is in the "rooms" colors, otherwise do it in post. you caneven create a new light source for both the subject and the room.
Additionally, add a shadow or shadows of your subject based on the "room" light positions and shadows. There are better, but this is an easy way for a beginner to create a shadow: create a duplicate layer of your subject and drop the brightness to 0 and contrast to full. or add a filter to color the subject all black. Then adjust the layer using a plug in or using key points to move and warp the black subject layer so that it appears as a cast shadow from the room light... set opacity and lots of blur to your liking to polish it up a bit. You can also warp it when necessary so that it changes direction on furniture and when it hits walls etc...The last thing i would add is Depth of field. For Example, if the actor is in Focus, and you are shooting on a 50mm lens, the wall behind him might not be in focus.
In your particular shot you might want to add a real table in the near field that covers some of the subject and has some simply things on it. a couple books, a bag, etc, maybe todays left over dishes... you can shoot the table as a video on green screen and add the layer to the scene in post. again, play with depth of field and camera position. perhaps even starting focus on the near field object and "racking" focus to the subject. This focus pull can be done completely in post..
You can play with the Depth of Field all sorts of ways. If the subject walks closer to camera, the back ground might fall further out of focus.
You can also add a foreground object to help the believe that the subject is in the room. Add a coat rack very very close to camera, out of focus just barely in frame, or add a bit of a chair back that might be in the room between the camera and the subject, or a plant, or even a bit of a "wall" or "door frame".... These sort of near field items will help to PLACE the subject inside a room and help build that 3-d real world you are trying to achieve.
Shooting your scene with the Real camera matched to the Virtual camera, adding a bit of the room color to the subject and adding a shadow to the subject, setting a bit of field depth, and adding something to near-field with proper depth of field, will greatly enhance the believability.
cheers
geo