[This story contains spoilers from the series finale of the Big Bang Theory.]

CBS' The Big Bang Theory ended Thursday just as it started.

Television's longest-running multicamera comedy in history wrapped after 12 seasons with the sense that life goes on for its beloved characters.

For Leonard (Johnny Galecki) and Penny (Kaley Cuoco), it meant new adventures (parenthood). For Sheldon (Jim Parsons) and Amy (Mayim Bialik), it meant personal growth after they won the Nobel Prize in physics and each handled it in different ways (by regressing socially and getting a makeover, respectively). And for others — married couple Howard (Simon Helberg) and Bernadette (Melissa Rauch) and perennially single Raj (Kunal Nayyar) — it really was about being content with the lives and families that they've built.

The Big Bang Theory has never been a show that plotted out its storylines well into the season, let alone a series finale. But Thursday's two-part series finale found a way to provide a fulfilling sense of closure as well as emotionally satisfying moments that illustrated its characters' slow and steady personal growth and development. What's more, producers also paid off several of the comedy's long-running jokes (the elevator!) and sneak in one final geek-tastic cameo.

Below, Lorre, former showrunner and exec producer Steve Molaro and current showrunner Steve Holland talk with The Hollywood Reporter about saying farewell to The Big Bang Theory, those big surprises and why they opted for such an emotional farewell. Click here to read THR's interviews with the cast about how their storylines ended.

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How long was this the ending that you had in mind? Was this always the idea?

Lorre: I had a couple of different things in mind, but this was far better than anything I could have presented. Credit for this finale has to go to Steve Holland and the writing staff. They proposed a finale that was centered on emotional closure in the characters' lives, in their relationships and in them growing as individuals and in their lives expanding. I was struck by how elegant that was and I jumped on board. It was better than anything I might have had in my head. And while there is quite a bit of story with the Nobel Prize and the pregnancy elements, it's a lot of character material there that took the finale in a different direction. It made the finale about the characters as opposed some big cataclysmic ending or everybody packing their bags and moving out and going different places. There is nothing wrong with that approach but this felt much more rewarding for us. I love the way it ended.

Holland: Knowing it was the end, we'd been talking about where we wanted to see these characters land. The show has never been plot-heavy and, for us as fans of these characters, it didn't feel right or necessary for us to do a heavy finale like that and put a "the end" stamp on all these stories. It was important to feel like their lives and their friendships went on. That was our starting place of how we wanted to build this finale.

There was a sense of life going on for these characters, who return home and have Chinese food in the apartment to a great scene with an acoustic version of the show's theme song. Was there ever a point where you were tempted to offer a more resolved conclusion for these characters?

Molaro: No. I mean there were bigger goodbye ideas that floated up here and there but the biggest philosophical drive that we landed on pretty early was this show makes a lot of people happy, these characters make a lot of people happy, they make us really happy. We wanted to know that tomorrow morning they were going to drive to work together and have dinner together and while the show was saying goodbye, the characters weren't. Philosophically, that was the most important part of the finale rather than putting giant bows on everybody's storylines.

The finale focused on Sheldon's growth — specifically, how in the past he'd take two steps forward and one step back. And after winning the Nobel, he very much regressed before having this massive breakthrough on the biggest stage of his life.

Holland: We knew about the Nobel Prize, and we knew that's what we were driving toward. And it did feel like a moment where Sheldon could regress a little bit because this is a thing he has wanted since he was a kid. So, of all the moments to make all about himself, it felt perfectly natural that he would try to hold so tightly onto this moment as being about himself. He realized at the end that it wasn't about himself, and he took that moment that he has wanted his whole life and, instead of turning the spotlight on himself, he turned it on this group of people without whom he would never have been where he was. He would never have been who he was. It felt like a great way to honor this whole ensemble and the journey of this show.

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Knowing that there were a few times over the years where you weren't sure if the show would come back for additional seasons, how much has this finale changed over the years?

Holland: The finale has certainly changed. If we had talked about it in season six, it certainly is a different finale than it is now.

Molaro: If we knew it was ending last year, the show certainly could have ended on the Sheldon and Amy wedding. I'm glad that it didn’t and I'm glad that we got to go as far as we did. But there have been stopping points that would have felt natural in some way over the years.

Outside of Sheldon and Amy's wedding, were there any other episodes or moments in the finale that you'd had earmarked for the series finale over the years?

Molaro: I don't know how much we ever really spoke about a finale until we knew it was coming for real. We were in the fortunate position to know pretty early on in this season and had a goal in mind as we thought about how we wanted to land it.

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'Big Bang Theory' Series Finale: Leonard and Penny's Big Surprise

And there were a couple red herrings this season with Leonard and Penny briefly discussing if they wanted to have children and considering moving away. Were you tempted earlier this season to explore what pregnancy would be like for Leonard and Penny? Walk us through the decision to do that in the series finale.

Holland: Leonard and Penny had been talking about having children — and the decision not to — this season.Their relationship was really the jumping off point of this whole show and it felt important to us to honor that relationship. I love them together and I wanted to know that they were ending in a happy place. Steve [Molaro] and I talked about it and he is the one who pitched that moment and that surprise revelation in the finale. It felt like a great way to land these characters and like such a surprise coming off what their storyline had been.                         

Molaro: I like that we were able to jump over a lot of the pregnancy tropes and learn after the fact that Penny is two months pregnant. We did that [two-month] jump in time and she's still wrapping her head around it because Penny wasn't sure how she felt about it. I'm really happy that we were able to get in a direct nod to the pilot in Sheldon's final speech, too.

Courtesy of CBS

Right! Sheldon says in his Nobel speech, "their babies will be smart and beautiful" — which is what Leonard says to Sheldon in the pilot after he first meets Penny. That was but one nod to some of the show's inside jokes that you paid off in the finale, including the elevator working, why Leonard always used to wear hoodies, seeing Howard and Bernadette's children for the first time and the return of Amy's famed tiara. Was there anything else you wanted to fit in but couldn't, like Howard's father or Penny's maiden name? 

Holland: The shape of the way the elevator opened was going to come differently when we were originally talking about it. The reveal of the Leonard and Penny's pregnancy was going to happen a bit differently. As we got into the thick of breaking these stories out, those pieces evolved and I think the way they landed is way better than the way we started talking about them.

Courtesy of CBS

Amy, after winning the Nobel, decides to get a makeover. Why was that something you wanted to explore now? Mayim said this was entirely the writers' idea.

Holland: When you have a long-running show, you want the characters to change but you can't push them too far because you still have to keep doing episodes week after week. But coming to the end, we could really start to push these characters. Leonard had some big changes in the last few episodes [including forging a new relationship with his mother and clearing that baggage] leading up to the finale and this felt like a great change for Amy's character, to give her that moment. When we went to Mayim and we asked, "How would you feel about getting your hair cut, because we have this story thought?" She got so excited and asked if she could cut her hair! She couldn't believe we were serious. It worked out very nicely. 

Courtesy of CBS

Raj and Anu broke up in last week's episode and he wound up with a meet-cute with Sarah Michelle Gellar. Why leave his storyline so open-ended?

Lorre: With Sarah, it felt like a wonderful way to add some comedy to what was otherwise a very serious scene at the awards ceremony. Because of the way this finale was presented by Steve Holland, we didn't feel like it was necessarily to have some big story closure for every character. That felt forced and somewhat unnecessary because sometimes big things don't happen to everybody all at once unless you're forcing it into a series finale. And that didn't feel realistic. It didn't feel like a comfortable way to go. So Raj's story just remains open.

Holland: So many ideas were talked about; everything was fair game and on the board going into early this year talking about what the finale could be and Raj finding his person was certainly one of them. We just felt we didn't need to tie up everything in a bow. Their lives were still going on. We gave him a fun and emotional closure in episode 22 with him and Howard [at the airport when Raj is tempted to move to London for Anu]. It felt OK to leave him still searching for love — and I believe that he ultimately will find it someday. but we didn't feel the need to have to show it.

Molaro: And I think this show started with these four guys all single. To honor the bones of the series, it's OK that somebody still hasn't found the right person.

Courtesy of CBS

How did the Sarah Michelle Gellar cameo come together?

Holland: When we were breaking out the finale and knew that they were flying to Stockholm, we knew that Raj wouldn't have anyone sitting next to him. Chuck thought it was a fun opportunity to have him sit next to someone fun and that it would be a good chance to do something fun here at the end. As we started talking about who that could be, we landed on Sarah Michelle. I have been a huge Buffy fan for years. I've rewatched that show probably more than any other. And a lot of the writers are big Buffy fans, too. It seemed like a cool cameo that felt real to this world and to these characters. Chuck spoke to her on the phone and at the beginning [of the call], she said her family loves Big Bang Theory. Then it was just a matter of working out scheduling.

Is there a theme you hope viewers take away after watching the finale — and ultimately, of the series as a whole?

Lorre: From the very beginning, the goal was really simple: to cause laughter. That's why we made the show and why we wanted to build a show around these characters. I love the characters because, despite how intelligent they were, they felt left out. They felt like outsiders looking through the glass at life going on without them. They were outliers — and that's a very common feeling; even the queen of the prom might feel like that in private moments. One of the things we did, probably by accident but then it became something we were conscious of, was these characters ate together all the time, which is what a family used to do. And that's really inviting. One of the things that was really inherent to the success of Cheers was that these people weren't related to one another but they clung to each other like a family. That happened on the Big Bang Theory. The characters aren't related but they behave and operate like a family even to the point of making each other miserable. But no matter what, they created a surrogate family. That underlies the comedy and may ultimately be more important than the jokes.

What do you see as Big Bang's legacy?

Lorre: I couldn't speak to its legacy. I'm too close to it. I just know that I loved being a part of this. I'm proud and grateful for it. I'm deeply sad that it's over. As to its legacy, I'll let that be for other people to decide — if that's a thing you decide about a situation comedy.

Holland: Its legacy isn't up to us. I hope that it has moved the culture of science in a good way. Ultimately, I hope that it's a show that brought people a lot of comfort and laughs and that they can look back on it or revisit it in reruns and get those same warm feelings again.

Molaro: I would like to think that this show managed to move the needle even just a little bit on the tolerance of anybody who is different.

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