For an entire generation of children, The Parent Trap created one extremely convincing illusion.

Lindsay Lohan had to have a twin.

How else could Hallie Parker and Annie James stand across from each other, argue, whisper, scheme and eventually hatch an elaborate plan to reunite their parents?

The two girls looked identical, but they did not feel identical. Hallie was the confident California kid raised by her father. Annie was the polished London girl living with her mother. They had different accents, different mannerisms and completely different personalities.

Yet both were played by one young actress making her feature-film debut.

And during some of the movie’s most complicated conversations, Lohan did not even have another actor’s face to react to.

Instead, she was sometimes performing opposite something far less glamorous.

A tennis ball.

There Was Only One Lindsay Lohan

Released in 1998 and directed by Nancy Meyers, The Parent Trap introduced audiences to Hallie and Annie, identical twins separated shortly after birth when their parents divorced.

Neither girl knows the other exists until they accidentally meet at summer camp.

The discovery produces some of the movie’s funniest scenes. They compete against each other, become enemies and are eventually forced to share an isolation cabin after their rivalry gets out of control.

Then they notice the similarities.

Matching birthdays.

Matching halves of the same photograph.

The truth finally becomes obvious: they are sisters.

Lohan’s performance made the revelation so convincing that many young viewers came away believing Disney had cast actual twins.

There was a good reason for the confusion.

The filmmaking tricks were sophisticated, but Lohan also had to create two characters distinctive enough that audiences could immediately tell who was who.

She was only 11 during much of the filming process, turning 12 shortly before the movie opened in theaters.

Decades later, Lohan has revealed just how strange filming some of those twin scenes actually was.

Imagine Having an Argument With Nobody

When two actors perform a conversation, they normally have each other.

One actor delivers a line.

The other watches their expression, hears the timing and reacts naturally.

Lohan could not always do that.

During scenes requiring Hallie and Annie to appear together through split-screen techniques, only one version of Lohan could actually be photographed at a time.

That meant she occasionally had to look toward an empty space while pretending her identical sister was standing there.

To make sure her eyes were focused in the right direction, filmmakers gave her a simple reference point.

A tennis ball.

Lohan recently recalled that she would look at the ball while performing one side of the conversation.

But that solved only half the problem.

She still needed someone to talk to.

The Other Twin Was Inside Her Ear

While Lohan looked toward the tennis ball, she could hear the dialogue belonging to the other sister through a small earpiece — sometimes called an earwig.

That allowed her to hear the missing character’s lines and respond at exactly the right moment.

Then the process could essentially be reversed.

Lohan would change characters, costume, accent and attitude and perform the other half of the scene.

The footage could later be combined to create the illusion that Hallie and Annie were occupying the same room at the same time. Lohan has specifically recalled working with both a tennis ball and an earpiece while making the movie.

Think about what that required from a child actor.

She had to remember where an imaginary person was standing.

She had to maintain the correct eye line.

She had to respond naturally to dialogue coming through her ear.

And she had to remember that she had already performed the other half of the conversation as an entirely different character.

The final scene could look effortless.

Making it was anything but.

There Was Also a Secret Second Girl

The tennis ball was not the only trick.

For scenes in which audiences saw one twin from behind, or when the twins needed to physically interact, the production also used a body double.

Her name was Erin Mackey.

Mackey provided Lohan with a real person to work opposite in certain situations and could stand in for one of the sisters when her face would not be visible to the camera.

The technique allowed filmmakers more freedom than simple split-screen photography alone.

If Hallie was facing the camera while Annie’s hair or shoulder appeared in the foreground, for example, Mackey could occupy Annie’s position.

Lohan could then perform against an actual person.

But whenever both twins’ faces needed to be clearly visible, the technical challenge became much greater.

The filmmakers had to combine separate performances while making sure the camera position, lighting, timing and Lohan’s movements lined up convincingly.

None of that would have mattered, however, if audiences did not believe the acting.

Hallie and Annie Had to Feel Like Different People

The most impressive trick in The Parent Trap may not actually have been visual effects.

It was Lohan herself.

Hallie and Annie do not merely have different clothes.

Hallie is casual, bold and unmistakably American.

Annie is more reserved, sophisticated and speaks with an English accent.

Even when the twins switch places — with Hallie traveling to London and Annie heading to California — Lohan had to perform one character pretending to be the other.

That created another layer of acting.

She was not simply playing Annie.

She sometimes had to play Hallie attempting to behave like Annie.

For a performer making her first major movie, it was an unusually complicated assignment.

Yet Lohan has said that playing the twins felt surprisingly natural.

When reflecting on The Parent Trap years later, she explained that acting could actually feel easier when she was younger because she approached it with greater freedom and experimented without overthinking every decision. She contrasted that experience with playing Anna Coleman in Freaky Friday, where she had to imitate Jamie Lee Curtis after their characters switched bodies.

As an adult, Lohan believes actors can become more self-conscious.

As a child, she simply played.

That instinct may be one reason the illusion worked so well.

Even Nancy Meyers Saw Something Special

Director Nancy Meyers recognized Lohan’s unusual screen presence early.

Meyers has recalled seeing an audition tape from New York and immediately sensing something special in the young actress.

The decision proved enormously important.

The Parent Trap became Lohan’s breakthrough and began a run that would eventually include movies such as Freaky Friday, Mean Girls and Confessions of a Teenage Drama Queen.

But for many fans, Hallie and Annie remain the roles that defined her first.

Nearly three decades later, Lohan still talks warmly about the movie, and she has said she would consider revisiting the twins if the right project came along and Meyers were involved.

The Trick Is Harder to Unsee Once You Know

Watch The Parent Trap today and the technology is easier to understand.

You know there is only one Lindsay Lohan.

You know some shots required a double.

You know other scenes were assembled from separate performances.

But knowing the trick arguably makes Lohan’s work more impressive.

During a scene that appeared to show two sisters arguing face-to-face, one of those sisters might not have existed yet.

There was no second Lindsay waiting to deliver her next line.

No identical face reacting from the other side of the room.

Sometimes there was simply an 11-year-old actress, an earpiece and a tennis ball positioned where her imaginary twin was supposed to be.

Millions of viewers saw two girls.

Lindsay Lohan had to imagine the second one.