In a recent interview with Glamour, Jim talks about the Shenny kiss.
"It was really fun—and, very honestly, really easy. Very early on working on this show, I began working a lot with Kaley on a lot of different scenes because Penny and Sheldon had a lot of scenes together, and it was a very easy relationship to be a part of on-camera.
Obviously, as different as she and I are—certainly as different as the characters are—that may not seem like the most logical thing to say, but as performers, she and I work very easily together. The first time we rehearsed [the kiss], it was literally just like an extension of the verbal dance we've been doing for eight and a half years.
It's always awkward to a degree, but this was shockingly easy. We had so much fun. And it was so much fun to do it live in front of the audience. I kept thinking, 'When a moment like that is going on, they're going to know it's coming and start laughing before we do it.' You have one shot to do it right in front of an audience like that, and they didn't at all. It was dead silence, and the vibe in the room was, 'What is happening?!' It was exactly what it should have been and then BAM, we were making out!"