And would the positive emotions brought on by playing the game clash with those in the content?
I think they would cheapen them.
I find that the capacity of the medium to make you feel present in a world is enough to amplify the positive emotions of the content, as much as the "negative" ones like fear or sadness.
As a home made case history the happiest, more joyful moments I remember of my wife playing a game are:
- chasing the jumping rabbit-like animal in Proteus
- watching the girl in white dress in the Path play with one of the younger sisters
- approaching the best points of view from which to watch the scenery in Dear Esther