When Star Trek: The Original Series' first pilot, "The Cage," was shot down by NBC, there was no guarantee the series was going to get off the ground even though it was given a second chance to prove itself with a new pilot. Jeffrey Hunter, the actor who played Captain Christopher Pike in the original pilot, elected not to return for another shot at the show so a new captain had to be found, and William Shatner was cast.
And while some actors who worked with Shatner on the set complained about his need to be in front of the cameras as well as other issues dealing with ego, one writer, David Gerrold, who penned the famously popular "The Trouble with Tribbles" came straight out and said that Star Trek: The Original Series and the movies were successful because of Shatner.
In The Fifty-Year Mission The First Twenty-Five Years by Edward Gross and Mark A. Altman, Gerrold was reported as having said that "all of the movies and all of the episodes hold together because Shatner holds it together."
Gerrold pointed out that Spock needed someone to play off of, and that was Captain Kirk. When Spock had scenes that didn't involve the captain, he didn't think they were interesting.
"The scenes where Spock doesn't have Shatner to play off of are not interesting. If you look at Spock with his mom or dad, it's very ponderous. But Spock working with Kirk has sthe magic and it plays very well, and people give all of the credit to Nimoy, not to Shatner."David Gerrold
While I agree that the scenes between Kirk and Spock are magical, I don't agree that the scenes with Spock and his parents are less so. In "The Journey to Babel," Spock's interactions with both his mom and his dad are wonderfully played with intensity. When Amanda slapped her son across the face, it wa just the right shock factor.
Shatner and Nimoy played excellently off one another, and DeForest Kelley was also a strong asset to the team. The Original Series wouldn't have been the same without any of them. Even today, it's difficult to find that type of chemistry between actors, the chemistry that made The Original Series as special as it remains to this day.