How can I remove the background of a video if the background is still?
In your sequence, place a still frame of the background with your footage place above. 


  
  V2: Footage
  V1: Still frame of background
  


In the Effects panel, find Video Effects > Keying > Difference Mask. Drag this effect onto your footage (the top-most track). 



In the Effects Control panel, set the 'Difference Layer' to match the track with the still frame.



Hide the still frame (bottom track) using the 'Toggle Track Output'. The background will now be transparent.



Inconsistencies of lighting, shadows and movement of the background will have a big effect on the quality of the outcome. 

Avoid casting shadows on the background by keeping the scene well lit. Be aware of objects in the background moving. 

If necessary, use a combination of color keying (a wall with a solid colour that contrasts with the subject) as well as masks to hide parts of the footage where no movement occurs.