Researching informal gameplay can be challenging, since as soon as a formal study design is imposed, it becomes neither casual nor self-motivated. As a case study of a non-invasive design, we analyze publicly posted gameplay videos to assess the effectiveness of a physics educational video game on special relativity. These videos offer unique insights into informal learning through gaming, as players do not only describe the gameplay mechanics, but also explore physics concepts in a think-aloud fashion while they ponder the experience and effects. We find that while this methodology has substantial limitations, it is complementary when it comes to assessing motivations and attitudes, as well as to gathering data on conceptual hurdles.