It's Barry Allen's [Grant Gustin] birthday in this week's Flash episode, "It's My Party, and I'll Die If I Want To." To celebrate, Team Flash is throwing a killer surprise bash. Attendees include old friends, familiar faces, and one nefarious uninvited guest. That party crasher comes courtesy of Dr. Ramsey Rosso [Sendhil Ramamurthy], aka Bloodwork, a former friend of Caitlin Snow, who is obsessed with immortality and "saving humanity" at the expense of others. This time around, Bloodwork manipulates Kid Flash [Keiynan Lonsdale] to sideline Barry and extend his reach over the Multiverse. The only thing he didn't count on? Oliver Queen (Stephen Amell).

Last seen in the Arrow series finale, the Emerald Archer used his godlike Spectre powers to alter history and lived happily ever after in a paradise dimension with Felicity. But with the very Multiverse at stake, the Green Arrow returns to give Barry a helping hand and raise his spirits. Amell recently spoke to CBR about reprising his role as Oliver, cheesy dialogue, tender moments, and Arrow's legacy.

Green Arrow and the Flash stand together in Season 9 of the Flash

CBR: Showrunner Eric Wallace called you to pitch Oliver's return. What about it checked off all the boxes for you?

Stephen Amell: First and foremost, I liked that it wasn't the series finale. Series finales get to be a little busy, and it becomes expected that you are going to have all these returning appearances and characters. I really liked that it was a random episode. I also appreciated how collaborative he was. I had a pretty particular idea of how I wanted the story to go. Obviously, I am not the one who puts pen to paper or starts typing. But it was just very nice that he was willing to be so collaborative because a couple of the ideas I had that ended up in the episode weren't in the original pitch.

At one point, Oliver states, ''I miss this.'' Was that something you could relate to? How much have you missed this character and universe?

I really have. The decision to finish Arrow when we finished, I stand by. I actually do think that was the right call. But it's like anything. You start to miss it. It had been a little over three years. When I got to Vancouver, it was in January. Absence makes the heart grow fonder. I stand by my decision, but it was really nice to be back.

Most of Oliver's interaction takes place with Barry. In what ways is this episode a love letter to that friendship?

We took all of our favorite moments, whether it be me pulling a fast one on him or telling him to calm down, or us having a beer at the end of the episode or fighting side by side. We wanted to stay true to the story, but we wanted to play the hits.

Related: The Flash Reportedly Features a Major Arrowverse Cameo

Green Arrow, The Flash, and Speedy stand together in Central City

Speaking of those hits, how was it suiting up in Arrow's trademark attire again?

It was very familiar. Let's put it that way. It was the same suit I wore in Season 8 of the show. It still fit. Honestly, it was the closest I had been to being annoyed with anyone on the production while I was up there. They insisted I go for a fitting on the suit. I was like, ''You already fit it to me.'' I think they just wanted to make sure I hadn't lost or gained a bunch of weight in the time I had off.

There are memes and clips all over the internet of Oliver declaring, ''You have failed this city.'' How fun was it uttering those words again?

It was amazing. It was really cool. You try not to get too cheesy but also give everyone a little bit of cheese.

Oliver hugging Barry and John choked me up. Were there any moments, on or off camera, that hit you?

The first scene that I filmed was the scene after all the action, where they are at the party. I will say I was actually quite nervous, believe it or not. We had to move around the schedule a little bit, and I ended up working a Friday. I wasn't really anticipating working until Monday. I think I was a little nervous. I got to share a nice scene with David Ramsey. I just wanted to do a good job. But no. I have learned to control my emotions when I am acting.

How do you feel Oliver has changed since viewers last saw him? He almost comes across more like Zen and Obi-Wan Kenobi.

They did a great job of writing the character in that direction. I wanted to convey more of a calmness and an easy-going nature, an all-knowing nature. I wanted it to feel like there was a natural growth because we hadn't seen the character in three-plus years.

Stephen Amell as Oliver Queen/Spectre in the Arrowverse Crisis event.

Brandon Routh, Ryan Reynolds, and now you are proof that you can play multiple heroes. What would it take to get you back in the DCEU or even the MCU?

I don't know. That would be a slightly challenging thing for DC. I still look exactly like the character that I played for close to 10 years now. You never know about these things. I am somewhat hesitant to pine about theoretical jobs in the future, only because whenever something pops up in my life -- be it Arrow or Heels -- by the Tuesday or Wednesday when it is hitting the trades, that Monday, I have never heard of the project before. I love the genre. I've said this before, but there might be more fun stuff to do with Oliver Queen. So we will see.

In retrospect, how would you sum up the Arrowverse experience? You kicked off this wave of superhero shows. What are you most proud of?

I just lean into our show. This has become a very popular genre on television, and it wasn't a popular genre or didn't have a great track record when we premiered in 2012. I think we accomplished a lot under very challenging circumstances. As we moved through the seasons, the expectations were that the episodes would get bigger and better and grander in scope. In reality, the longer you go, the quicker you are making them. We did an awful lot when sometimes there wasn't a lot to work with, and we should be proud of that. I think we really defined an era on television, the entire Arrowverse, and not just our show.

Looking ahead, any updates on the Code 8 movie or the second season of Heels?

The update is both should have updates very soon. Code 8 is almost done. It should be delivered to Netflix. That's happening imminently if it hasn't already. We anticipate releasing a trailer and a second season of Heels very soon.

The Flash Season 9, Episode 9 will air on The CW on Wednesday at 10 PM EST.