(Think Stretch Armstrong.)
EW caught up with Cavanagh to get the scoop on the big hour.
Ive put 97 percent of the time and energy of this season into that episode.

Credit: Jack Rowand/The CW
Its the classicFlashwith gunplay and explosions, romance, fighting, comedy.
Ho, ho, ho.
So 404 is filled with that kind of stuff.
Elongated Man alone should give you some preview as to what were doing.
If you might imagine this is an episode where the tone ofThe Flashmarries Jim Carrey.
Im excited to be in charge.
Its very hard to take a man, [and stretch him].
And he was like, Well, you guys have been doing this [for so long].
I was like, No, dont take that mindset.
You dont want to come on and by your third episode go, Now Im finding the guy.
You want to come up gangbusters and steal every scene it’s possible for you to find.
I said, Heres an example: Pick three episodes of H.R.
Wells last year and watch them.
Im like, Yes, sir.
He couldnt stand HR Wells, some smooth-talking idiot.
Now theres another handsome boy coming in here prancing around and Harry doesnt cut into chains that well.
We hadnt really seen that.
I dont think we had seen this world and darkness that affected The Flash the way it did.
As the guy telling that story, it felt like a real privilege.
This will be a much more typicalFlashand you just do the best job you could.
I realized that one was dark and this one is light.
What we want on this one is levity weve never seen onThe Flash.
It felt like a privilege of equal amount, but in a very different fashion.
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.
The combination of Hartley and Danny is a phenomenal element.
Were playing the entire field there.
Youre never going to know what youre going to get, especially when youve got an established movie star.
Hed call me coach.
He was like, What do you need from me, coach?
Literally, anything that I gave him, he was game for.
I suggested a little thing where hes a proud investigator and we created this 40s noir vibe.
The Flashairs Tuesdays at 8 p.m.
ET on The CW.