I just interviewed Jack Dorsey and Dick Costolo. Here's what they had to say...

In a bit of a surprise, Twitter CEO Dick Costolo announced on Thursday afternoon that he would be stepping down as CEO effective July 1.

In his place, Twitter founder, and board member Jack Dorsey will become interim CEO.

I got on the phone to talk with them about why Costolo is stepping down now, and what Dorsey plans to do.

Based on my conversation, it sounds like Dorsey is interested in being CEO of Twitter, full time. I asked him directly, twice, if he wanted to be full time CEO. And he didn't say no. He just said he was focused on his job as interim CEO.

"I'm not focused on that question at all," Dorsey said. "I'm focused on making sure we continue our momentum and to amplify what we're doing."

For what it's worth, The New York Times reports, "Ever since his ouster in 2008, Mr. Dorsey has wanted to return to the helm of Twitter, according to two people with knowledge of his thinking."

It would make perfect sense for Dorsey to become CEO. He founded the company, he has always aspired to be the leader of a big consumer focused technology company, a la Steve Jobs. He's currently the CEO of Square, but that has pivoted to be more of a small business focused company.

As for Costolo, he says he was ready to move on. He says he put together a great team that was clicking and he felt like the company was never in a better position.

While that may be true, Costolo was under a lot of pressure as CEO. Twitter's user growth rate had slowed, and the company missed revenue estimates last quarter.

Twitter has 300 million monthly active users, which is sizable, but is dwarfed by Facebook which has 1.4 billion active users. Instagram now has over 300 million monthly users. With Snapchat gaining fast on Twitter, it's not hard to see a world where Twitter is suddenly the fourth or fifth option for advertisers looking to spend money on a social platform.

Investors were growing impatient with Costolo, which likely quickened his departure.

While Costolo's ending on a rocky note, the truth is that he did a good job in his time as CEO. As he said to me, he took a company with no revenue valued at a billion dollars and helped steer it to billions in revenue, a $24 billion valuation, and a successful IPO.

Here's the full transcript:

Business Insider: So Dick, on the call before, you said that you started having these conversations with the board at the end of last year. And then it picked up some more steam around February, and then you decided to make it public. Can you give me any insight into what the thinking was, what precipitated this at the end of last year that caused you start thinking about moving on?

Dick Costolo: Yeah, I told a few, a couple of members of the board at the end of last year. At the time, I only talked to, you know, a couple of them, and said, next year, if we feel like the team is really in the right place and we're executing the strategy and doing the things we said we're gonna do, I want to start, you know, thinking about what's next in life for me. So let's keep an eye on that and check back in in the February board meeting and see where we are. You know, then that was on the heels of our analyst day investor conference in November, where we were all, as a leadership team, getting excited about the way the way that strategy was coming together, the way the team was starting to gel.

And I remember Kevin Weil at that conference got up, and said, and boldly declared, you're going to see a pace of execution from us early next year that you haven't seen from the company before. And people were taken aback by that, but that was the kind of confidence that we were starting to have in the team. At the February board meeting, after the board meeting, I got together with more of the directors in the immediate aftermath of that. And said, "I think we all now really like the way the team is working together. I sure do. They're forward right with each other, they engage directly with each other in debate, they solve problems together, there are no sharp elbows." Not only that, Alex and Kevin on engineering and product are starting to build up a next level of leadership underneath them. This was a conversation that you and I had even had a month before at CES.

So I was excited about the progress we were making, and said, listen, at the summer board meeting in June, when we're, trying to come to some, you know, to figure out where we are, and if we still feel great about this, then let's move forward.

That meeting was last week. And in the aftermath of the board meeting, we all, the board sat around the table for a few hours and had this discussion and, stuff like, the team has never been stronger, ever. The org is stable and robust and the pace of execution and quality of execution against a clear strategy that we all believe in has never been better. And on the heels of Fabric's immense growth in its last year and our Periscope acquisition that's working so well, there's not going to be a better time than this to move forward with this transition.

BI: The story of Twitter has been one of upheaval, turmoil, and change. If you believe the team is strong, things are gelling, doesn't that throw it all asunder? You're bringing in someone new. Doesn't that put a lot of pressure and stress and more shakeups into a team that's had a lot of that through the years?

DC: Well that would be true if it was someone from the outside, but we have the benefit of Jack being the chairman of the board, the inventor of the product, and a cofounder. He's already a visible leader within the company, he speaks to the company and in company events both in the US and abroad with some frequency. There's no one better than him to lead the company through a transition like that.

He's close to my entire leadership team. I felt the scrutiny of the company would intensify if I remained CEO while we kicked off a process like this. As such, we all felt it would be a distraction to the company, and we didn't think it was in anyone's interest to have that distraction. So bringing Jack in while we embark on this process to give the board the search committee of the board the time and space to do this with the right person leading the company.

BI: That would make sense if Jack in fact were sticking around. Jack, do you want to be the CEO? Do you plan to stick around? The New York Times is reporting, according to people familiar with you, that you have angled to be more prominent, you've longed to be returning to Twitter and run it. So is "interim" just a thing to hold us, or are you really just interim.

Jack Dorsey: My focus is on interim, and my focus is on making sure we continue our momentum around all of our products and initiatives. And that we continue our pace of delivery, and facilitating a smooth transition as the board conducts a search for a permanent CEO. We do ahve a search committee comprised of Peter Currie, Peter Fenton, and our cofounder Evan Williams, and we're going to take the time to pick and choose the right CEO for the company. I'm not focused on that question at all. I'm focused on making sure we continue our momentum and to amplify what we're doing.

BI: Do you want to be the permanent CEO?

JD: Again, I'm not going to answer that because it's not my focus, I'm not what I'm thinking about, I have enough to focus on.

BI: I'm curious, if the momentum is great and everything's good and the strategy is not going to change, then why do this now? What about this job did you not want to do anymore that you decided to make this change?

DC: It's not that. I came to Twitter in August of 2009, I was asked to come in as the COO. There was only about 60 people here, tens of millions of users, one office, zero dollars in revenue, valued at $1 billion. We were just doing a round as I got to the company that Insight Venture Partners led that valued the company at a billion dollars.

It's grown to $24 billion, or whatever it is today, 4,000-some people, hundreds of millions of users, and dozens of office around the world. I've accomplished, you know, an extraordinary amount and despite all that there are still way more items on my list of hopes and dreams and potential for the company. It's never gonna be - and for no great leader it should ever be - completely the case that you have more accomplishments than you have hopes and dreams for the company. There's never gonna be a perfect time. And I decided at the end of last year that the important thing to do was get the team into a robust and stable place, with a strategy in place that we loved and were executing against. And adding into all that the ability to have been able to have brought Periscope into the company, one of the most compelling, new, powerful native mobile video apps that's really off to the races. I just thought I wasn't going to find a more perfect time than this.

BI: In the Valley, there's a fixation on founder-led companies. I think one of the criticisms you got a lot, Dick, was that you didn't have the vision for the product, and the vision for where to take this product and where to take this company. Jack you're going to step in and execute that. Do you feel like you have a better idea as the founder of this company, the person who had originally had to sort of take it to the next level and see where this product can go?

JD: I'm coming into the company as interim CEO with a strategy and a direction that Dick's set forth with his team and I believe in that. So I don't consider it founder and non-founder, it's really about the direction and I am extremely confident in it.

BI: One last question you want to answer Dick - You want to tell us what you honestly think about Chris Sacca and what he's been saying lately?

DC: You know what, I think that it's great that we work at a company that people are so passionate about. If we didn't, that would be a terrible thing. It's because of Twitter's role in the world, its potential, remarkable social, socioeconomic, and cultural, global impact that people feel so strongly about it and they have very specific ideas about what should be done. Everybody should be working at a company like that.