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Tom Cavanagh may have been MIA for the early part of the season, but he’ll be pulling double duty during Tuesday’s episode of The Flash as he steps behind the camera to direct.

It’s a big hour for The Flash: In addition to featuring the debut of Danny Trejo as Gypsy’s menacing father Breacher (more on that here), the episode also introduces Hartley Sawyer as Ralph Dibny, a.k.a the Elongated Man (more on that here), whom Cavanagh likens to a Jim Carrey-esque character as Team Flash comically fails to get his newfound powers under control. (Think Stretch Armstrong.) EW caught up with Cavanagh to get the scoop on the big hour.

ENTERTAINMENT WEEKLY: Harry was MIA at the top of the season, but now you’re pulling double duty this week and also directing the hour.

TOM CAVANAGH: I’ve been back on this show for a couple of episodes, but I’ve literally spent the entire time shepherding along the story of 404, because I’m prepping it, directing it, acting in it, editing it. I’ve put 97 percent of the time and energy of this season into that episode. It’s the classic Flash with gunplay and explosions, romance, fighting, comedy. It’s interesting because Carlos Valdes and I are always looking to find ways to accentuate moments with humor — not humor as an aside, but rather humor that comes organically out of a moment, which sounds like an obnoxious thing to say, but think about movies like Butch Cassidy and the Sundance Kid, where it’s like, “I can’t swim.” “Are you kidding, the fall will probably kill you!” Die Hard: “Now I have a machine gun. Ho, ho, ho.” The stakes were already established and the humor was in the moments as opposed to beside the moments. What’s interesting about this season is we’re making an attempt to follow that template much more than we ever have done in the past, and as somebody who’s tried to push that agenda in season’s past, it’s lovely to have people encouraging that movement. So 404 is filled with that kind of stuff. There’s even a Die Hard homage, Princess Bride, there’s a lot of fun things in there that I think that the fans are going to enjoy.

And what will we be seeing in the episode?

This is where we introduce Hartley Sawyer as Elongated Man and he does a spectacular job. “Elongated Man” alone should give you some preview as to what we’re doing. If you can imagine this is an episode where the tone of The Flash marries Jim Carrey. I had the green light to do the things — I freely admit I’m not above being shameless on camera — and I had the green light to extend those proclivities as far as I wanted. Not that it is going to be devoid of the action or spectacle and the heart that Flash always carries with it. We definitely will deliver those moments, but in addition to those moments, there’s some freelance levity that we’ve rarely seen. I’m excited to be in charge.

It’s very hard to take a man, [and stretch him]. It’s like The Incredibles. His stretching power opens a world of possibility up, and so your stretching man also needs to be a Jim Carrey-like figure I think to really sell that. You could play it cool, but if you play it cool, you’re going to be missing so much opportunity. And so my thing I said to Hartley was, “It would be a tragedy for you to come on and be such a big part of our show now and to defer to us.” And he was like, “Well, you guys have been doing this [for so long].” I was like, “No, don’t take that mindset. You don’t want to come on and by your third episode go, ‘Now I’m finding the guy.’ You want to come up gangbusters and steal every scene you can find.” I said, “Here’s an example: Pick three episodes of H.R. Wells last year and watch them.” He came in the next day after watching three episodes, he’s like, “Oh, I didn’t know you could do that on this show.” I’m like, “Yes, sir. That is yours for the taking, that area that we don’t fill is yours to fill, so go fill it.” And he did. He did marvelously.

How does Harry feel about Ralph?

Personally, having Hartley play such a massive part in the episode — and basically how Hartley played was how the episode was going to play essentially — so I’m overjoyed at having Hartley Sawyer on board, but Harry Wells can’t stand him. He couldn’t stand HR Wells, some smooth-talking idiot. Now there’s another handsome boy coming in here prancing around and Harry doesn’t cut into chains that well.

When it came to directing, what did you want to do this time around that you didn’t get a chance to do when you directed “Once and Future Flash”?

All that is incumbent to the script. The last one I directed, I liked it for the very reason that it felt like a departure from the normal Flash episodes because we jumped eight years into the future, and we had that kind of post-apocalyptic landscape to contend with. We hadn’t really seen that. I don’t think we had seen this world and darkness that affected The Flash the way it did. And to create these skeleton selves of what these characters and sets were, and stuff that viewers of The Flash are familiar with, to rip all that up so it could be portrayed eight years in the future, that was a tremendous opportunity that I felt. To be able to explore that darkness and to have Grant play the long-haired dark and defeated version of The Flash, to have Carlos further his eccentricity, it was a boon to be involved with. As the guy telling that story, it felt like a real privilege.

When Andrew Kreisberg and Greg Berlanti asked me to direct this one, [it was like], “Because this is the funniest Flash we have ever attempted, we want you to do it.” I prepared myself mentally by saying, “Listen, you’re not going to get as extravagant an opportunity or script the way you did last time, because that was something we hadn’t done. This will be a much more typical Flash and you just do the best job you can.” And then it ended up being the funniest Flash we have ever done, almost the polar opposite of the previous one. I realized that one was dark and this one is light. What we want on this one is levity we’ve never seen on The Flash. It felt like a privilege of equal amount, but in a very different fashion. The idea that we could have sanctioned humor on an hour-long television show is just something I’ve always gravitated to. The quickest way to align your audience is to make them laugh. Here we have a green light to do that. It was a wonderful opportunity and I loved every minute of it.

Was there anything you were shocked you got to do in this episode?

I wouldn’t say shocked at all, but there was a marriage of pathos and humor that we’ve rarely seen. Here’s an example: We have two guest stars in this episode that are basically the opposite tact from one another. One is the Jim Carrey-esque character of the Elongated Man that Hartley plays, and then Danny Trejo comes and starts blowing stuff up from his Earth. The combination of Hartley and Danny is a phenomenal element. We’re playing the entire field there. We’re going from goal line to the other goal line, to use a sports analogy here — we’re covering the entire field. Trejo, also, for what he’s known for, the humor that he brings in this one is [great]. You’re never going to know what you’re going to get, especially when you’ve got an established movie star. He’d call me coach. He was like, “What do you need from me, coach?” Literally, anything that I gave him, he was game for. It was just a delight to work with a guy like that, and Hartley was the exact same way. I said, “We can’t go too far. If we go too far in reaching for the humor, I can pull you back, but you’re elastic, so your reach should exceed your grasp. Go for it!” He was game. I suggested a little thing where he’s a proud investigator and we created this ’40s noir vibe. What if this guy sunk so low that he’s got an entire drawer dedicated to donuts and they’re not fresh, they’re half-eaten? None of the guys down in L.A. had seen it, but the first thing I get from Kreisberg was, “This was amazing! A donut drawer!” The idea that we could push the boundaries of the humor and find a receptive audience, not only with our producers, but hopefully with the audience, it felt like — what we wanted to do this year was make it lighter and it felt like we were well on the road to accomplishing that task with this episode.

The Flash airs Tuesdays at 8 p.m. ET on The CW.