Sitting in / on that chair

I think "sitting on" the right one but I couldn't get how is "sitting in" is also right.

I would accept "in" as right if it is used as "sitting in living room". Am I wrong?

I think "sitting on" the right one but I couldn't get how is "sitting in" is also right.

I would accept "in" as right if it is used as "sitting in living room". Am I wrong?