Leonard, Sheldon, and Penny's first encounter in The Big Bang Theory was apparently impromptu, according to Jim Parsons. Despite the show concluding its 12-season run over four years ago, it continues to garner immense popularity, with dedicated fans rewatching the series through streaming platforms and new viewers discovering it.
When The Big Bang Theory concluded in 2019, it featured seven main characters. However, it was evident from the start that Sheldon, Leonard, and Penny were the true focal points. Their initial introduction to each other, which Parsons discloses in Jessica Radloff's The Big Bang Theory: The Definitive Insider Story Of The Epic Hit Series, was not originally scripted. See his complete quote below:
How Sheldon, Leonard, and Penny’s Meeting Scene Laid Big Bang Theory’s Foundation
: The omission of the full exchange of greetings and farewells from the original script of The Big Bang Theory is quite remarkable given the scarcity of such moments in the show's history. Nevertheless, the scene continues to amuse me whenever I reflect upon it. It was a wonderfully peculiar and yet straightforward moment that conveyed a great deal.The inception of The Big Bang Theory can be traced back to Leonard and Sheldon's visit to the sperm bank, a concept that was retained from the original pilot at the request of CBS. However, this particular scene has since been removed from reruns as both Chuck Lorre and Jim Parsons felt it was incongruous with the overall tone of the show. Instead, Lorre has stated that the genuine commencement of the comedy lies in Leonard and Sheldon's first encounter with Penny in their apartment.
Lorre's decision to remove the sperm bank scene is understandable given its impact on the overall narrative. The trio's initial meeting holds greater significance as it establishes the foundation of the show. Given the initial challenges faced by Lorre and Prady, the convergence of Johnny Galecki, Kaley Cuoco, and Parsons in their respective roles provided reassurance that they were heading in the right direction with the show's revamp. Additionally, this pivotal moment effectively conveyed to viewers the dynamics and expectations of the trio's relationship.
While Amy may have married Sheldon, his most crucial bond in The Big Bang Theory was the friendship he shared with Penny and Leonard. Without them, the socially-awkward genius would likely have ended up alone. Regrettably, CBS devoted an excessive amount of time to Sheldon's storyline, resulting in the underutilization of Penny and Leonard towards the end of the series.
Source: The Big Bang Theory: The Definitive Insider Story Of The Epic Hit Series