Pinsent and Mercer are both Newfoundlanders, hardly the Canadian mainstream. But, being one step removed, may make them better at portraying Canadians.
Keanu ACTs Canadian, as Jim Carey, for example, doesn't.
How about Mike Myers, or Peter Sutherland?
Lorne Green or that dude that did Perry Mason?
Was it Cleese that did the lumberjack?