Paul Naschy can be seen as a member of a picnic group who get to bounce Boris Karloff (or rather his double) into the air using a blanket as a makeshift trampoline.
The documentary The Man Who Saw Frankenstein Cry (2010) got the title due to an anecdote that happened in this shooting. Here's the text that was shown at the end of the documentary: "At the end of the 60s, the I Spy (1965) TV show crew arrived to Spain in order to film some episodes; in one of the Boris Karloff had a role. Among the extras that they had looked for this episode, they found a young man who was hiding that he was trying to get on in cinema from his parents. His name was Jacinto Molina (Paul Naschy). At the end of the working day, Karloff was waiting for the car that would take him to his hotel. He did so for hours while cold and night were adhering to him. While Jacinto was looking from the distance, some tears appeared on the actor's old face. Nobody else was witnessing that moment. That day, Jacinto became the man who Saw frankenstein cry."