I didn't watch movies. I've played an enemy force a few times in military exercises doing exactly that. It is shocking how easy it is to just pass as a civilian in a busy city environment, observing and picking your place and moment so you can have an easy out. You "shoot" a guy when they're not looking your way and then most of the time if you picked your spot right they don't even know where it came from before you're already walking away looking like any other bystanders. Of course, using those tactics means you're not covered by the Geneva convention if ever captured because you're not wearing a uniform. But I don't think an American military invading Canada would give a shit about international law to begin with.

Ultimately, one tactic does not exclude the other either.