After six seasons and more than 100 episodes, "Pretty Little Liars" will finally reveal its mysterious Big Bad in tonight's season finale -- and according to series star Sasha Pieterse (Ali), you don't want to be late. Because A's big reveal is happening at the start of the game-changing finale.

"It's almost immediately. We jump right into it," Pieterse told MTV News ahead of Tuesday night's big reveal. "We're not hiding anything anymore. There's no questions that I have, as far as our past goes and everything that has happened in the show thus far. We literally cover everything in this finale."

"What I really appreciate about this episode is that we're not just throwing you things, so that you guys are satisfied," she added. "There's actually a really deep, dark story line here, and you're seeing a totally different version of everything that has happened so far. It's this crazy person's life, and it shows what's making this character tick and why all of this madness has happened. And it's actually kind of heartbreaking."

Ali was the first Liar to come #fAcetoface with Charles DiLaurentis -- or the person who's been parading around as Charles this entire season -- at the end of last week's penultimate episode, but she won't be the last. We know all of the Liars will eventually meet their terrorizer in the season finale, and by the episode's end, they'll finally understand the true reason behind A's sick games.

"It's so suspenseful, but it's also very emotional," Pieterse said. "We finally see this character for who they are, and it's a cool twist."

"It's not a cut and dry thing," the actress added, noting that fans might feel empathetic towards the show's #1 villain by the episode's end. "You're not sure how to feel about it. How could you feel so much for this character who's done so many terrible things? That's such an intense part of this episode."

For Pieterse, filming the episode's big reveal and its shocking aftermath -- a twist six years in the making -- was thrilling. Not only were her own questions answered (A is "someone Ali knows," Pieterse confirmed), but she got the chance to work with the actor playing A without the mask, which is a BFD in Rosewood.

"It was shocking! It's not what we're all used to doing, to have it all out in the opening was really weird," she said. "And then having these intimate scenes with this character, who we've never had a face for, was really cool for us. We love this person... who I can't say the name of [laughs]. But it makes sense, and it fits this person so well -- they've totally made this character their own. It was an honor to be part of that and watch this person work."

With A's identity out in the open, does that mean all of our lingering questions will be answered? According to this PLL, the finale will delve deep into the dark past of the show and answer, once and for all, the questions that have haunted fans from the beginning.

"All the dirty laundry is out there, honestly," said Pieterse. "The DiLaurentis family is crazy, and now you're going to understand why. Ali has really been in the dark this whole time, and she's going to learn a lot of things about her family in this episode. There's so many things she doesn't know that she now does -- and it changes everything. It's intense!"

If you say so, Ali.