I agree that vulnerability is a key Culp acting skill. It's the main aspect of the character that attached me to Bill Maxwell, that in your face Fed FULL of insecurities and vulnerabilities. The writers developed Maxwell much more than Ralph, probably due to the complexity of the Fed and Culp's capacity to bring so much more depth to the character, which the stories and Culp himself veered to vulnerability so often. Kelly's vulnerabilities were apparent, too, mostly through his emotional instabilities and his resultant drinking abuse, and it always intrigued me how it put him, Scotty and his country at risk, at times. But, it's still a huge draw to me-- I mean, it's a perfect recipe for a fan:
1. Combine a cup of tall with two cups lean and handsome--stir rapidly until character winds up with shirt off
2. Add in intense episode whereby main character is emotionally hurt
3. Add in a pinch of masterful expression of vulnerability
4. Put whole mix on TV and ensure pause and rewind button on remote are in working order.