“It’s the longest goodbye ever,” Andrew Rannells tells ET

about the final season of Girls,

which started filming in May. The Broadway star plays Elijah alongside Allison

Williams (Marnie), Jemima Kirke (Jessa), Zosia Mamet (Shoshanna), Adam Driver

(Adam), and Alex Karpovsky (Ray) on the Lena Dunham-created HBO series. “So, I’ll

be doing that through the summer.”

In January, it was announced that Girls would officially end after six seasons, with the final

episodes expected sometime in 2017. “I conceived Girls when I was 23 and now I’m nearly 30 -- the show has quite

perfectly spanned my 20s, the period of time that it’s about -- and so it feels

like the right time to wrap our story up,” Dunham, who plays Hannah, said at

the time. “We look forward to creating a sixth season that will honor our

amazing cast, crew and fans. And in the Girls

universe, nothing ever ends too neatly.”

MORE: Why Allison Williams Chose to Leave It 'All on the Field' on This Season of 'Girls'

According to Williams, things will sink even lower for

Marnie than they did in the standout, season-five episode, “The Panic in

Central Park,” which saw her character unexpectedly reconnect with a drug-addled

Charlie (Christopher Abbott, who left the show after season two) and end her

marriage with Desi (Ebon Moss-Bachrach). “We’re going to get there,” she teases.

“I actually finally know when it is now, and it’s coming up in our shooting

schedule. There’s a definitive bottom for Marnie.”

With that said, the actress says she doesn’t know how the

series ends -- yet. “I can’t speak to that specifically,” Williams says, adding

that Dunham and the entire cast and crew are focused on making a final season

that lives up to the legacy of the series, especially following particular

praise for its penultimate season. “We are working on making it excellent

enough to withstand that level of attention. We've always been working that

hard, but there's kind of this acknowledgement that we need to make sure we

leave on a good note.”

MORE: How Jemima Kirke Became More Than Just One of the 'Girls'

Season five saw many of the girls (and guys) grow up, and

learn to see life outside of themselves. Yet, it still remains unclear where

the story will go during its final episodes. (Will any of them find love, success

or satisfying work?) But that doesn’t have the cast worried. “I trust Lena so

much. She has such a clear vision for all these characters,” Rannells says. “I’m

just excited to see how it all ends.”

“There is some stuff I’m looking forward to,” Kirke adds. “But

I can’t really tell you.”

One thing you won’t see in the final season is Jessa’s

parents, or at least the return of her estranged father, whom the audience met

in season two. “They're not coming back. I think they're done with parents this

season,” Kirke says. “We've seen everyone's mom. We get it. We get where it all

comes from.”

MORE: How Jenny Slate Channeled the Bumps of Our 20s into a Surprise Return to 'Girls'

While the show is only two months into filming, for Williams,

it’s already an emotional experience. “Personally, it's very weird, the idea

that I'll never play Marnie again,” she says. “So, I'm just going to leave it

all out there because I can't really go back. I can't make any adjustments.”

“The finality of it is really strange,” Williams continues. “This

is the only version of my job that I know. I went from college to Girls graduate school, and now I'm like

a doctorate in Brooklyn angst and now, I'm ready to enter the other workforces.”