Not much is known about Sophia Bush's decision to exit Chicago P.D. last season other than that it was a decision she and the writers came to early on in the show's fourth season. Now, Bush has opened up about the details of her decision to leave and why staying attached to Chicago P.D. would have been unbearable for her.

According to Bush, even though snagging the lead role in a TV show was pretty much her dream, she knew by the end of Season 2 she couldn't keep doing that job. It took her another year to finally sit down and have the talk about leaving though.

"A year later, when I sat my bosses down — it was in the summer between seasons 3 and 4 — and I said, 'Here's where we are. Here's everything you're aware of. Here's how I'm coming to you today,'" Bush told Refinery 29. "'If something really drastic doesn't change, I'm leaving at the end of the year'..."

That's a pretty big move for a female lead to make, and Bush knew she had to go about it exactly the right way because of stigmas within the industry about female actors. Basically, she knew she'd be called a diva and labeled "difficult to work with" if she didn't go about having this discussion in a calm and collected manner.

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"I understand how the business works and how women are treated — I said, 'I'm giving you not two weeks notice and I'm not coming in here throwing s*** and breaking lamps and saying I'm never coming back,'" Bush says. "'I'm giving you 23 episodes notice. I'm giving you that much time. So there will be no conversation in which I was hysterical, emotional, in which I was being a quote irrational female or whatever you want to put on it. I'm literally sitting in front of you, like, cool as a cucumber. If this has to be, like, a big swinging d*** competition, I promise you I will win. But know this now: if we're not having a very different conversation by Christmas, then you know with 100 percent certainty in December that come the end of April, I'm leaving.'"

Bush is still keeping quiet about the details of what was going wrong behind the scenes and with who, but it's more than clear something pretty huge was happening to make her situation unbearable. "For me it felt like I was trapped in a burning building," Bush says. "I was just so unhappy and it was my dream job and I was miserable and I had to go."

Now that she's been freed from her contract with Chicago P.D., Bush has signed an overall deal with 20th Century Fox TV, which obligates her to star in an upcoming pilot, but also offers her the ability to develop additional projects as an executive producer.

Chicago P.D. airs Wednesdays at 10/9c on NBC.

Sophia Bush, <em>Chicago P.D.</em>Sophia Bush, Chicago P.D.