The makers of superhero films go the extra mile to please their audiences by slipping in a cameo or two from actors or celebrities who aren't in the film industry. This adds to the fun for viewers, not just in finding those moments, but also in discerning their relevance to the story, as a really creative director does more than just drop a known quantity into the movie for a fleeting moment. Sometimes an actor's presence is a sly nod to an earlier version of the property, perhaps in a different medium. Sometimes a singing star branches out a bit from music videos and performing on stage. And sometimes, a business mogul or a politician lends his or her presence to the proceedings.

Both Marvel and DC have dropped these kinds of surprises and sight gags into their films, with some credited, some not. Some are reminders of the storied past that comics films draw on. A few showcase special guest stars. And with re-releases of films on DVD, cable channels and online, viewers can discover early appearances of actors before they hit their greatest fame. Here are 15 celebrity cameos from the DC Extended Universe and the Marvel Cinematic Universe that have slipped under the radar.

15 ZACK SNYDER IN WONDER WOMAN

Zack Snyder in Wonder Woman

Fan buzz about 2017's Wonder Woman was high before, during and after its run in theaters, on its way to raking in $821 million in a worldwide gross. But among the crowd-pleasing performances by Gal Gadot as the title character and Chris Pine as Steve Trevor was a very subtle, hard-to-spot cameo by producer Zack Snyder.

The Hollywood Reporter revealed an image of Snyder dressed as a World War I British soldier, toting a Lewis gun. Snyder later posted the image with the caption "in the trenches with Wonder Woman." But it was harder to find the moment when he actually appears in the film, although it was right in front of everyone. Snyder is in the background of the grainy daguerreotype of Wonder Woman, Trevor, and her band of warriors.

14 TOM MORELLO IN IRON MAN

Tom Morello in Iron Man

Tom Morello, founding member of the rock bands Rage Against the Machine and Audioslave, dabbled a bit in acting in the late 1990s and early 2000s -- that is, apart from appearing in music videos. He is of course better known for the soundtrack music he provides for films and TV shows such as Blade II, Spider-Man and The Faculty. Still, as an actor, Morello has also shown up in Star Trek: Insurrection, and in an episode of Star Trek: Voyager.

But he surprised his fans with a message about his involvement with the Iron Man films. Morello tweeted, "Is that really me in Iron Man? Of course. Iron Man kicks my ass. I'm working on score for Iron Man 2. Rockin. Movie is awesome." In Iron Man, he is in military garb and appears as one of the guards holding Tony Stark captive.

13 ELON MUSK IN IRON MAN 2

Elon Musk in Iron Man 2

Child genius. Check. Tech innovator. Check. Investor in new technologies, including artificial intelligence, space flight, electric-powered transportation and solar energy. Check. No, it's not Tony Stark ... we're talking about his real-world analogue, Elon Musk, who is worth somewhere north of $20 billion. Among Musk's accomplishments are founding the forerunner of PayPal, and founding Tesla and Space-X.

Although the Marvel Comics character has old-school self-made men such as Howard Hughes as inspirations, for the movies, actor Robert Downey Jr. and director Jon Favreau consulted with Musk as a model for a modern mogul. So it is only fitting that Musk shows up in 2010's Iron Man 2 as himself. He is dining in a restaurant when Stark and Pepper Potts (Gwyneth Paltrow) greet him and make small talk.

12 OCTAVIA SPENCER IN SPIDER-MAN

Spider-Man cameos Sara Ramirez Octavia Spencer Lucy Lawless

Before winning the Oscar for Best Supporting Actress in 2011's The Help, Octavia Spencer racked up a long list of credits in various movies and TV shows. In 2002's Spider-Man, Spencer is in scene-stealing form. She's the jaded person behind the table checking in the contestants vying for the cash prize at a wrestling match that Peter Parker (Tobey Maguire) intends to enter.

She's less than confident that he can hold his own against the burly battlers, and lets him know it. "There's no featherweight division here, small fry. Next!" When he insists "Sign me up," she warns him that the wrestling league will not be held responsible for his probable injuries. Also, Sara Ramirez of Grey's Anatomy has a cameo as a uniformed New York Police officer, and Lucy Lawless is unrecognizable as a punk rock girl in a man-on-the-street interview.

11 EN VOGUE IN BATMAN FOREVER

En Vogue in Batman Forever

Batman Forever introduces viewers to Dick Grayson, played by Chris O'Donnell, whom Bruce Wayne has taken under his wing after his parents were murdered. Grayson has a long way to go to become a superheroic figure, when he discovers the Batcave and impulsively takes the Batmobile out on a joyride. The sight of the car causes a sensation on the streets in one of Gotham City's seedier neighborhoods.

A crowd quickly forms around the vehicle, but is disappointed when it isn't the Caped Crusader who emerges. One of four ladies of the evening shouts, "That ain't Batman!" Her friend rejoins, disgustedly, "That's Batboy!" The unsatisfied quartet who wanted to see a real hero were Terry Ellis, Cindy Herron, Maxine Jones and Dawn Robinson, the original members of the chart-topping, Grammy-nominated singing group En Vogue.

10 SEN. PATRICK LEAHY IN BATMAN MOVIES

Patrick Leahy Heath Ledger Joker The Dark Knight

You might be forgiven for thinking that Patrick Leahy represents the good citizens of Gotham, rather than Vermont, which he has done in the U.S. Senate since 1975. That's because Leahy moonlights in Batman films and shows, a lot. He's done five: two more than Christian Bale, three more than Michael Keaton, and four more than Val Kilmer, George Clooney or Ben Affleck.

Leahy did a cameo as himself in Batman Forever and in Batman and Robin. In between, he voiced the Territorial Governor in the Batman: The Animated Series episode "Territorial Governor." In The Dark Knight and The Dark Knight Rises, Leahy was a board member for Wayne Enterprises. In the latter film, he stood up to The Joker, who pulled a knife on him. For Batman v Superman: Dawn of Justice, he played "Senator Purrington," which we can hope means Leahy can be around for future films.

9 GARRETT MORRIS IN ANT-MAN

Garrett Morris in Ant-Man

In 2015's Ant-Man, Paul Rudd is burglar Scott Lang, who has a bewildering adventure after he steals what he thinks is just a motorcycle-rider's suit and finds himself shrunken to the size of, well, an ant. After a wild journey across town, Lang comes back to his full frame with a jolt, crashing on top of a taxicab and startling the driver.

That driver was Saturday Night Live vet Garrett Morris -- the first actor to play Ant-Man on screen. He did so in the 1979 sketch "Superhero Party," fondly remembered by Ant-Man director Peyton Reed. He and Marvel Studios reached out to Morris, who happily agreed, although he admits many might not understand his relevance. "People 25 or under, you don’t know what the hell [my cameo] means. ‘Why is that black man in there looking up? What’s he doing?" Morris said.

8 AIDY BRYANT IN AMAZING SPIDER-MAN 2

Aidy Bryant in The Amazing Spider-Man 2

If you're a tourist in the Big Apple, one of the things to do is go to Times Square and mingle with the various goofballs who make a little cash letting you pose for pictures with them. Many are in costume as various pop culture figures, although one, The Naked Cowboy, has a rather skimpy outfit.

In 2014's Amazing Spider-Man 2, the wall-crawler finds himself standing next to Aidy Bryant of Saturday Night Live, decked out as The Statue of Liberty, complete with green body paint. Her one line to him? "Nice costume." Bryant trolled star Andrew Garfield when he guest hosted SNL that year, interrupting his monologue to give him how-to tips. This included showing the clip of their on-screen moment together -- and claiming it was the whole movie.

7 NOEL NEILL IN SUPERMAN RETURNS

Noel Neill in Superman Returns

In 2006's Superman Returns, Lex Luthor, played by Kevin Spacey, is released from prison, thanks to Superman not testifying at his appeal because he was off-world at the time. Luthor swiftly proves he has not reformed from his evil ways by finding a rich widow and marrying her for her money. When she dies, he inherits her fortune and is off to pursue his latest scheme.

It is no ordinary actress who plays Gertrude Vanderworth, the ailing dowager Luthor seduces and scams. It was Noel Neill, the first actress to play Lois Lane on screen, in the 1948 serial Superman. She reprised the role in the sequel serial Atom Man vs. Superman and in the TV series Adventures of Superman, beginning with the second season. Neill also had a cameo in 1978's Superman, as Lois Lane's mother.

6 LOU FERRIGNO, PAUL SOLES IN THE INCREDIBLE HULK

Lou Ferrigno Paul Soles The Incredible Hulk

In the movies, the Incredible Hulk has been a creation of computer-generated wizardry. But for the small screen, the Hulk was put together the old-fashioned way, with body paint, wigs and tattered clothes applied to the massive six-foot-five-inch frame of former Mr. America and Mr. Universe Lou Ferrigno. From 1977 to 1982, Ferrigno was the title character on TV's The Incredible Hulk, through 82 episodes on CBS and three TV movies on NBC from 1988 to 1990.

Ferrigno was a security guard in 2008's The Incredible Hulk, and also did the creature's voice. But another character -- Stanley Lieber, the pizza shop owner -- was played by Paul Soles, who was the voice of the Hulk in The Marvel Super Heroes Show in 1966, and Peter Parker/Spider-Man in the 1967 animated series.

5 PAUL REUBENS IN BATMAN RETURNS

Paul Reubens in Batman Begins

The first person viewers see in 1992's Batman Returns is a man of aristocratic mien, a moustachioed gent with a monocle who wears a tuxedo as if it's everyday garb. We hear screams down the hall; his wife is giving birth, and, clearly, things are not going well. We skip ahead a few weeks or months -- it's not clear -- and our aristocrat is with his wife ... and the baby is in a cage. Then we see them take the baby for a walk in a pram, despite it being a snowy winter, and toss the pram off a bridge.

This is the movie origin of The Penguin, and the man we saw was his father, played Paul Reubens, best known as the character Pee-wee Herman. Reubens later played Penguin's father, Elijah van Dahl, on the Gotham TV series.

4 JEFFREY DEAN MORGAN IN BATMAN V SUPERMAN

Jeffrey Dean Morgan in Batman v Superman

We all know the moment: A happy family -- mom, dad and son -- is on a night out on the town. A gunman emerges from the shadows. A father steps forward to protect his wife and son. An automatic pistol spews bullets. A mother's pearl necklace shatters and beads fly everywhere. The gunman flees. And a boy's world is left in ruins.

Batman v Superman: Dawn of Justice played the moment one more time. This rendition, which opened the film, featured Jeffrey Dean Morgan as the father, Thomas Wayne. Morgan is no stranger to fans of superhero films, having played The Comedian in the film adaptation of Watchmen in 2009. Morgan has also been the bat-wielding Negan in The Walking Dead TV series since 2016.

3 BRUCE CAMPBELL IN SPIDER-MAN MOVIES

Bruce Campbell in Spider-Man trilogy

Fan-favorite character actor Bruce Campbell has been pals with director Sam Raimi since their days in high school. The early collaboration Within the Woods, shot on Super 8 film in 1978, was the prototype for the Evil Dead series, leading Raimi and Campbell to work together many times in the following years.

When Raimi was tapped to direct Spider-Man, he gave Campbell a cameo as the ring announcer who introduces Peter Parker to the wrestling arena -- although he bluntly rejects the name "The Human Spider" and calls him "the Amazing Spider-Man!" Campbell shows up in 2004's Spider-Man 2 as a theater usher who blocks Parker from seeing lady love Mary Jane Watson's (Kirsten Dunst) play. In Spider-Man 3 (2007), Campbell is the maitre'd at a French restaurant who tries to help Parker propose to Watson.

2 NICK OFFERMAN IN SIN CITY

Nick Offerman in Sin City

Until he hit stardom as Ron Swanson on the 2009-2015 NBC sitcom Parks and Recreation, Nick Offerman was a busy, if unheralded character actor in various movies and TV shows -- the kind of guy who toils away for a decade or more before becoming an overnight sensation.

One of his early roles was in 2005's Sin City as Burt Shlubb, a goon known as Fat Man. Shlubb is partnered with Douglas Klump, who is called Little Boy. The two are crooks for hire, called upon when someone needs muscle or to get rid of a body, no questions asked. As Shlubb, Offerman is unrecognizable, with a blonde buzz cut, a Van Dyke beard, multiple earrings and a nose ring, and a spiked dog collar around his neck.

1 FRANK MILLER IN ROBOCOP 2

Frank Miller in Robocop 2

Sin City is based on the works of writer/artist Frank Miller, who parlayed groundbreaking runs on Daredevil and Batman: The Dark Knight in the late 1970s and early '80s into a career in Hollywood. Miller wrote the story and was the scriptwriter for 1990's Robocop 2, and shows up briefly as "Frank, the Chemist."

Miller's experiences soured him on Hollywood, and he was reluctant to allow Sin City to be turned into a film. But director Robert Rodriguez was excited about the property, and won him over with the pledge that the film would be a "translation" of the comics property, not an "adaptation." In the movie version of Sin City, Miller is a priest who listens to Mickey Rourke's Marv in the confessional booth.