Why does Frank wear a rabbit suit?

A critical and theatrical flop, Donnie Darko made back only $1.2 million of the $4.5 million budget at the box office yet managed to build a dedicated cult following. The number of viewers who actually saw this film in theatres may have been small, but the fandom was strong. It left viewers swapping theories for weeks afterwards, because let’s be real, nobody really understands the plot which makes it not only intriguing but also very relatable to members of the cultural fringe of society. "Why are you wearing that stupid man suit” became an inside joke shared between cinephiles across the world.

In this DAZED article, actor James Duval reveals his iconic character was an easy one to create, saying it was probably the easiest acting job he’s ever had. The hardest part was capturing the dialogue, which had to be recorded in the studio as the iconic bunny suit trapped sound inside the mask. The article explains the method saying, “he would first record a line in a whisper, then re-record the same line in a gentle voice. Both recordings were then split into five different voice tracks that were layered to achieve this raspy vocal sandwich.”

On set Duval had to yell his lines while inside the suit which made it exceedingly difficult for actor Jake Gyllenhaal to hear – making the chemistry between man and bunny suit even more impressive.

Frank’s bunny suit lives on through memes, fan art and Halloween costumes still to this day two decades later.

Let’s not forget to mention the iconic soundtrack featuring Australian alternative rock band The Church with their infectious tune ‘Under the Milky Way’.

The supernatural force that appoints Donnie to end the alternate universe adopts Frank because Donnie killed him previously. As a manipulated dead, Frank knows what is happening and his job is to guide Donnie on his path to correcting the timeline of the primary universe by returning the jet engine to its original universe; Donnie is unaware of all this.

The costume is necessary because it's an essential part of Frank's character as a dead person (it's what he was wearing when he died) and also because it's so alien (not just some guy), Donnie will understand that he's dealing with things beyond normal.

As far as the costume being a creepy bunny, there is no significance in that. Frank was an artist and it was something he was rendering out when we see him in the ending scenes. It just adds a freaky ambiance is all.

Update: This article has some interesting suggestions on how you could interpret the Rabbit costume

What does the rabbit mean? That depends on how you interpret the movie. If you think that the film is a serious exploration of physical/metaphysical reality, you're apt to see Frank as a kind of rabbit-angel. His role is to guide a reluctant hero into becoming the instrument of God. In this reading, God wishes to save earth, and unfortunately (or not) this entails getting Donnie to commit criminal and destructive acts. Within this context, Frank's ugliness might be explained as the destructive side of salvation. Perhaps he is a monstrous rabbit in order to suggest that Donnie himself must become both prey/ victim and a kind of spiritual predator.

If you follow the parallel-universe theme but do not like the religious reading, you might see Frank as simply analogous to the White Rabbit from Alice in Wonderland. The White Rabbit had some scheduling concerns ("I'm late, I'm late, for a very important date") and led Alice down to a bizarre alternative world. Like the White Rabbit, Frank is under time constraints, having only 28 days to get Donnie with the program. And when he first leads Donnie out to the local golf course, scene of Donnie's first vision of him, one could say that Frank has inserted Donnie into the spiral of a new time/space dimension, just as the White Rabbit did for (to) Alice. In short, rabbit hole in Alice in Wonderland is wormhole in Donnie Darko, and both access a new reality.

If you're inclined to see Donnie as just a very disturbed teenager, then Frank is a tad more malicious. He embodies the dark, destructive side of Donnie, imagined as the flip side of the rabbit stereotype: ugly instead of cute, bizarre instead of familiar, destructive instead of reproductive, and so on. In this reading, Frank stands not just for the evil side in Donnie but in all of us.