I’ve been involved in a few playtests as a player (some for published products, some for things that never hit the market) and what I read in your link was pretty solid. His point about having others run the game is particularly on point. I’ve seen and been in several situations where someone provided objective perspective that those closest to the issue could not see.*
As a player, my personal approach was to ask and answer questions:
1) can I make PCs I’d like to play? To answer that, I’d design a half dozen or so before picking one to use in the actual playtest. If the system is strongly linked to a specific genre, I’d make some that were iconic to the genre and some that would be oddballs or even black swans.
2) are there parts of the design I thought were weak, ungainly, overcomplicated, poorly explained or otherwise problematic? To answer that, I’d try to create PCs that used those rules & mechanics enough to see how they performed under different situations. It wouldn’t necessarily be the PCs main shtick (though it might be), but it would be something the character would use relatively often.
3) were there exotic subsystems that might cause problems? Again, I’d try to aim a PC design at using such a subsystem, leaning into it as hard as possible to see how the rest of the system- and resultant gameplay- handled a raging corner case. If an X-Files inspired game has within it the possibility- however rare- of actual supernatural abilities, at least one of the PCs in the playtest needs to have them and use them.
* in my first semester of my first year of law school, I saw a student (for whom English was not her primary language) ask about an interpretation of a law that the professor had just completed working on with other learned minds for the state of Texas. Her question stopped his lecture dead in the water- her reading was diametrically opposed to how it was supposed to be read, but he nonetheless stood there mulling over her interpretation for quite a while. After several minutes of silence, he announced that, while her understanding of the law’s language was an unusual one, it was nonetheless a perfectly valid one. Because of that, he announced that- after that class- he had a BUNCH of phone calls to make so they could eliminate the ambiguity.
