Shannen Doherty almost didn't join the Beverly Hills, 90210reboot. The 49-year-old actress was the last of the original cast members to officially jump onboard the Fox series, BH90210, in which the stars played heightened versions of themselves.

In ET's exclusive, where the actress sat down with longtime friend Sarah Michelle Gellar at her California home, Doherty shared her true feelings about reprising her role as Brenda Walsh -- and what made her change her mind.

"I know you were originally hesitant to do the reboot, and I know that there might've been someone sitting right here that really forced you to do it because they wanted to see you in it," Gellar told Doherty.

"You're right, I didn't want to do it. I have my reasons for not wanting to do it," Doherty confessed. "One of them is I've played the character so many times and I've done so many versions of 90210 that it felt like we had exhausted that avenue. I believe less is more sometimes. You want to leave people wanting to see a show, as opposed to being like, 'Oh my God, I hope that never comes back on the air.' So, I was just a little trepidatious."

However, at the time of her having to make a decision, Luke Perry died and she got news that her breast cancer had returned as Stage IV. Those two factors made a huge difference in her decision making.

"Someone very important to me passed away and I got my diagnosis, and it was all sort of within a couple of months, and it just made me give pause and rethink it," Doherty admitted. "And yes, it was a way to honor Luke -- which was important to me -- but it was also a way to honor my cancer family by saying, Hey, yeah, yeah, yeah, like, nobody's gonna know, but at least when it comes out that I have Stage IV, I will have been able to say, 'Yes, but I worked throughout it.'"

Gellar added that she had so many friends who were crew members on the show tell her, "'Oh my goodness, Shannen is the most professional. These hours were so long.' And I was like, 'Yup, that's my girl. That's my girl.'"

Additionally, with October being Breast Cancer Awareness Month, the Stand Up to Cancer ambassador hopes to show others that Stage IV cancer doesn't have to be a death sentence, and with well-funded research and treatments, people can live a full and vibrant life.

Doherty added that she was glad she made the decision to reunite with her 90210 family.

"90% of everything that came out of my mouth was improv. None of it was scripted," she shared. "Brian [Austin Green] and I were, and Ian [Ziering] too, we were all just improving. To have that freedom that everyone allowed us to have, I can actually say that was an excellent experience. To actually be on a show where you can improv and be that creative and free [was great]."

BH90210 was short-lived, however, and didn't get picked up after its six-episode summer run.

See more of ET's exclusive sit down with Gellar and Doherty, below.

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