Canadian costs law generally exists to discourage people from bringing unsuccessful litigation, and, to a lesser degree, to discourage defendants from contesting successful cases. The basic rule therefore is that "costs follow the event" -- if you win, you get costs. Most provinces have additional rules that let people make formal offers to settle which, if they beat, they get their costs when they otherwise wouldn't, or, in the case of plaintiffs, they get extra costs.
Pre-trial applications generally follow a rule where costs go to the successful party "in the cause". In other words, to get your costs of a pre-trial application, you generally have to win both the application and the underlying law suit.
Oddly, though, the Supreme Court of Canada seems to like to give the costs of leave applications (equivalent of certiorari in the SCOTUS) to the successful appellant "in any event of the cause." That means even if they lose the ultimate appeal, the other side has to pay them for (some) of the legal cost of arguing the leave application (which is now done solely in writing).Two successful leave applicants got their costs that way yesterday.
This seems a bit perverse. A leave applicant lost in the Court of Appeal. If they also lose in the SCC, then they are 0-2 in appellate courts. At least half also lost in the trial court. So why do they get some of the costs that were necessitated by bringing the legal process to bear?
The SCC almost never gives reasons for leave decisions (which makes sense), so we don't know what the thinking is. Any former insiders are invited to post anonymously. Uninformed speculation is invited as well.