Playtest like you aren’t there

If you are too focused by what the player is doing during a playtest, they will notice it and play differently. They will be more aware of what they’re doing and the pressure of failing will rise. You’ll always have this problem, players will never play like they would on their own, in a safe environment. But you can relieve some pressure and make it less ‘formal’. Stand back from the game and player a bit more than usual, make it less obvious you’re there and it’ll feel less like you’re judging everything he/she does. So don’t seem too involved, but definitely keep full attention. Especially when the player says something or is having trouble.