"Sitting" is dubiously "an activity", so the distinction is pretty much non-existent, but if you're splitting hair, seated is a passive form, used in context of state, as opposed to activity. There is no practical difference although you'd rather write about lazy, comfortable way of sitting as "being seated", and attentive, firm, active as "sitting".
There is a significant difference though, if "he was seated by someone". That means he sat down there following someone's direction. You could say:
"We were seated in the first row of the theater by the usher"
It means the usher guided you to these chairs and asked you to sit there.
"We were sitting in the first row of the theater by the usher"
means the usher was sitting in another chair, next to yours.
The person may be implied. That means the original sentence still may imply someone seating "him".
He was led to the front row and seated in front of me.
And yes, it does create an ambiguity.
"You were seated by the exit"
doesn't mean the exit told you to sit, but that someone told you to sit next to the exit (or that you were just sitting there). And if instead of "exit" that will be "the guy selling pop-corn" it becomes quite unclear, whether someone told you to sit next to that guy, or that guy found you a place to sit.
To is traveling, until, till, and til are time. If you check their definition, depending on use, they're not interchangeable. Like to and too.
Best Answer
Considering that the definition for method starts with "A procedure..." there's not much difference.
So for all practical purposes they can be treated as interchangeable. That's not a very satisfying answer, though, so if there are slight hints of difference, they would be: