Depends on airline and airport. For some, even if online checked in, boarding pass needs to be 1st validated at separate "internet check-in counter". For 2 reasons -1st to cross-check with your passport, that you are the correct person travelling.
2nd, they even ask you to weigh before tagging "cabin carriage" (for more sticky airlines ie budget carriers).
Same applies for FAST check in machines.
At gate, some cursory spot checks with your boarding pass thing (I kena even by SQ). Then when physically boarding, some spot checks (again budget carriers).
See abuse with many lugging in heavy cabin carry. At expense of full overhead compartments (if you not 1st to board).
At Bkk's Don Muang (more of LCC terminal), I was surprised to be able to go straight to security and passport control
-with just web check-in printout. So much for security lapse