Are you one of those passengers who needs "spillover space"?

Or are you the person spilled over into? And what happens when two spillers over are seated next to each other? Or perhaps you're wondering if you count as a spiller. Southwest Airlines makes it clear for you:

“Customers who are unable to lower the armrests (the definitive boundary between seats) and/or who compromise any portion of adjacent seating should proactively book the number of seats needed.”