On one of the ends, most likely.
People just seem to avoid the middle seat of a sofa, probably because they feel less grounded there.
A bolster is a tube-shaped pillow. Set against the sofa back, it supports the small of the back. Set along the joint between the seat cushions, it defines the middle seat and gives the person who sits there more of a sense of enclosure and private space. Coordinate the bolster with the sofa's throw pillows, and it won't look like an afterthought. Some content on this site has been supplied by paid sponsors.