First of all, Anthony Rapp was already playing the role of a teenager opposite Kevin Spacey every evening on stage, which called for Kevin Spaceys character to briefly climb on top of the teenage character. They would have rehearsed or acted that scene dozens, maybe even hundreds of times together,
From what I understand Anthony Rapp was an impressionable gay teenager with a little crush on Kevin Spacey, who managed to get himself invited to one of Spaceys parties. This is no different from how any of us felt as teenagers, in my case I had crushes on several male teachers in high school. Anth