From the drug-fueled "Wolf of Wall Street" to the genteel "Great Gatsby," it’s safe to say Leonardo DiCaprio has the range and talent to play any personality.

Now, for his next big role, he plans to play 24 of them.

DiCaprio has reportedly signed on to produce and star in “The Crowded Room,” a movie based on the true story of Billy Milligan. In the late 1970s, Milligan – a young man from Ohio who had been indicted on three counts of rape, kidnapping and aggravated robbery – was diagnosed with a condition experts then called multiple personality disorder. According to a psychologist, Milligan had approximately two dozen identities; two of his alternate egos had committed the crimes, and therefore Milligan was unaware of his actions during the time of the wrongdoings.

Milligan’s case made headlines after he was the first person to successfully use multiple personality disorder as a defense in court. He was acquitted on grounds of insanity, although he spent the remainder of his life in psychiatric hospitals until his death in 2014. His diagnosis, however – which has since been renamed “dissociative identity disorder,” or DID by the medical community – helped launch the once-obscure condition into common parlance.

Of course, Milligan’s story isn’t the only time the notion of a “split personality” has been explored in literature, film and popular culture. “The Three Faces of Eve,” a 1957 film adaptation of a book about a patient with dissociative identity disorder, won a then-unknown Joanne Woodward an Academy Award for best actress. Showtime recently aired “The U.S. of Tara,” a television comedy-drama in which Toni Collette starred as Tara, a woman with three alternate identity states. Sally Field’s breakout dramatic role was as a teacher with DID in the drama “Sybil.” And everyone knows the story of the mild-mannered Dr. Jekyll and his evil alter ego, Mr. Hyde.

However, misconceptions about DID linger, experts say. Here’s what you need to know before watching “The Crowded Room” – or any movie or TV show that portrays DID.

What Exactly Is Dissociative Identity Disorder?

Bring up DID, and you’ll most likely receive the following reactions: “Wait, isn’t that when there’s multiple people inside someone’s body?” “Aren’t people with DID crazy?” And perhaps the most common question: “Does DID even exist?”

The answer to the first two questions is “no,” says David Spiegel, associate chair of psychiatry and behavioral sciences at Stanford University School of Medicine.

Long story short, Spiegel says, DID is a disorder that tends to stem from severe, early-childhood trauma – often from extended physical or sexual abuse, likely inflicted by a parent, neighbor, teacher or other authority figure. As a coping mechanism, patients with DID “dissociate," meaning they lose connection with their thoughts and memories. Their personality splinters off into separate identities, or alter egos, allowing them some respite from their emotional pain.

These separate identities coexist at different times, causing disruptions in memory and consciousness, along with bouts of amnesia. One minute, someone with DID is saying something; the next, another element of his identity emerges and he brings up a new topic midstream. Typically, there’ll be a noticeable change in mood or manner, although experts say the process is nowhere near as dramatic or abrupt as portrayed on television or in movies.

Sometimes, a patient’s personality states, called alters, can even have their own history, names or self-perceptions. They might be aware – or unaware – of one another’s existence. Spiegel compares it to someone acting differently at work than she would at a party. A person with DID, however, won’t be able to maintain a clear sense of memory or self during these switches.

Although the condition was once called “multiple personality disorder,” Spiegel was one of the parties responsible for changing its name to “dissociative identity disorder” during the fourth revision of the Diagnostic and Statistical Manual of Mental Disorders, more commonly known as the DSM. The name change, Spiegel says, was made to convey the disorder’s reality. Patients with DID, he says, don’t have a host of personalities inside of them, clamoring to emerge at any given time. Rather, they lack a cohesive and continuous identity.

Despite its inclusion in the fifth DSM, DID is still considered a controversial diagnosis in the medical field. Some prominent figures, including Paul McHugh, former psychiatrist-in-chief for Johns Hopkins Hospital and its current distinguished service professor of psychiatry, deny DID’s existence. They say it’s an invention of psychologists, and that the disorder has no scientific grounding. Often, skeptics bring up the real-life case of “Sybil,” the DID patient whose life inspired a book, movie and TV miniseries. Critics later dissected her story, deeming her condition a mishmash of confabulations, or false memories.

Other sources, like the International Society for the Study of Trauma and Dissociation, have no doubt that DID is a legitimate disorder. They estimate it affects about 1 to 3 percent of the population.

“I think there are some people who are reluctant to diagnose it because it has been controversial,” says Philip Kinsler, former president of the ISSTD and a clinical associate professor of psychiatry at Dartmouth University’s Geisel School of Medicine. He adds that many mental health professionals know little about DID, and aren’t necessarily trained to diagnosis it.

According to psychologists who study the disorder, there is scientific credence to DID’s existence. Studies suggest DID is documented in countries as far-flung as New Zealand, Japan and Turkey. And functional magnetic resonance imaging, or fMRIs, reportedly show different patterns in the brains of patients with DID.

How Is DID Diagnosed and Treated?

According to experts, well-developed screening tests help clinicians diagnose DID. However, it typically takes years for most patients to receive a diagnosis. Physicians often mistakenly think patients with DID have schizophrenia or bipolar disorder. Many patients have no idea they even have DID.

“They’ll say, ‘People tell me I did things that I don't remember doing,’ or ‘I find clothing that somebody else bought,” Spiegel says. Often, they experience depression, anxiety or symptoms of post-traumatic stress disorder. It’s also common for them to have flashbacks or nightmares, or engage in self-injury.

Patients diagnosed with DID are typically treated with therapy. Clinicians tend to elect an “eclectic” approach, meaning they use a wide range of techniques to tailor treatment for the patient. “You have to go back and deal with the early childhood trauma and attachment problems and understand what drives so much of their current symptoms and behaviors,” says Bethany Brand, a professor of psychology at Towson University who specializes in trauma and dissociative disorders. Cognitive behavioral therapy, in particular, can help patients distinguish between the past and present, allowing them to calm their brain during times of stress or flashbacks. Other therapies used to treat DID are inside-oriented therapy and dialectical behavioral therapy.

The overall treatment goal is to integrate the patient’s identities, which isn't always possible.

What Are Some Common Misconceptions About DID?

Although Billy Milligan is one of the most visible figures in popular culture with DID, he’s not at all representative of the average DID patient, Brand says.

“With DID patients, if they feel hostility or aggression they take it out on themselves with self-harm,” Brand says. “They’re self-destructive and repeatedly suicidal, more so than any other psychological disorder. So that's what's typical – not this wild aggression, or stalking women [or robbery].” Lacking a conscience, she says, is more consistent with another condition called anti-social personality disorder.

Another misconception is that “alter” states in DID are as defined and separate as, say, Toni Collette’s in “The U.S. of Tara.” Although a small subset of patients experience what clinicians call a “florid presentation,” a majority of people with DID experience much more subtle transitions between their various identity states.

“Most of the time, when people are simultaneously struggling with a warring side of them, it's just not that obvious to other people unless there's some kind of crisis when it’s going on,” Kinsler says. “It’s not, ‘One minute I’m Jack, and the next minute, ‘I’m Suzy.’ Most people are fighting this war within their heads.”

And while many DID patients have trouble establishing relationships or maintaining a career, there are individuals who are high-functioning and successful. Many are married and have families. Clearly, there’s no “typical” life path for a person with the disorder.

“I went to school, and I did really well,” says Olga Trujillo, a recovering DID patient and former practicing attorney who once worked for the U.S. Department of Justice. “I played sports, and I had friends. I seemed normal except for the fact that no one really knew me.”

Trujillo was abused as a child and diagnosed with DID in her early 30s. Her journey wasn’t an easy one: Her marriage dissolved, and for years she struggled with flashbacks and symptoms of post-traumatic stress. Now, she says, she makes it a point to be open about her disorder.