What is the difference between the two?

a) I will be sitting down on the chair.
b) I will be sitting in the chair.

Thanks in advance.
Kamo Emotion: smile
Please sit on the bench.
Please sit in the chair.

"on" and "in" distinguish the shape.
you can use either prepostion and the real meaning of your phrase is not changed, but is it slightly more approprate to use "on" if the surface is flat, like a classroom chair, bench or stool, and "in" if the chair has deep arms and cushions ..!
Students: Are you brave enough to let our tutors analyse your pronunciation?
To sit on a fence? Is on the correct preposition? What does it exaclty mean?
I think it means one doens't participate in some event.
When I was in Ireland, I saw a sign saying: "Home-made Ice-cream, sit in or take away!" ***
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Pls send me the detailed differences between on and in.