Being an actor involves just that: acting. However, despite the many personalities these talented people bring to life, we can't help but form our own ideas about them as people. A particularly beloved role can carry forward and affect how we see their other appearances - for example, the weird feeling watching beloved comic Robin Williams as a creepy old dude in One Hour Photo.
Before they were famous - or simply in roles 'out of character' or under the radar - many actors who are lovely people in real life took on the hardest, most evil roles of all: they played sick real-life serial killers.
Check out 7 actors who dared to go to the darkest place - and took on the skin of real-life killers.
1. Charlize Theron - Aileen Wuornos
Movie: Monster (2003)
The Killer: Aileen Wuornos killed 7 men during her tenure as a prostitute. She was given the lethal injection in 2002.
Less Evil Roles: Mad Max: Fury Road, Aeon Flux, Prometheus
2. Neil Patrick Harris - Paul Kenneth Keller
Movie: Not Our Son (1995)
The Killer: Known as America's most prolific serial arsonist, Paul Kenneth Keller set over 30 fires, killing at least 3 people and causing over $30 million in property damage.
Less Evil Roles: How I Met Your Mother, The Smurfs
3. Jeremy Renner - Jeffrey Dahmer
Movie: Dahmer (2002)
The Killer: Jeffrey Dahmer raped, dismembered and kept the bodies of 17 young men in the 1980s. He's possibly America's most famous cannibal.
Less Evil Roles: The Avengers, Hurt Locker, Bourne Legacy
Check out the lovely Jeremy Renner as the totally not-lovely Dahmer...
4. Michael Rooker - Henry Lee Lucas
Movie: Henry: Portrait of a Serial Killer (1986)
The Killer: Sure, Michael Rooker often plays morally questionable roles, but none so vile and depraved as Lucas, who tortured, raped and killed women.
Less Evil Roles: The Walking Dead, Guardians of the Galaxy
Check out the lovely Michael Rooker as the totally not-lovely Lucas...
5. Laura Prepon - Karla Homolka
Movie: Karla (2006)
The Killer: Homolka shocked Canada when she assisted her husband in the rape and murder of at least three women - including her own sister, Tammy.
Less Evil Roles: That '70s Show, Orange Is the New Black
Check out the lovely Laura Prepon as the totally not-lovely Homolka...
6. Hayden Panettiere - Amanda Knox
Movie: Amanda Knox: Murder on Trial in Italy (2011)
The Accused: Amanda Knox was convicted, then acquitted of the murder of student Meredith Kercher in Italy. The trial gained an enormous amount of media attention do to Knox being female and attractive.
Less Evil Roles: Heroes, Scream 4, Nashville
7. Ryan Gosling - Robert Durst
Movie: All Good Things (2010)
The Accused: Although Gosling's role is 'David Marks' in the movie, the story is a direct retelling of the case of Robert Durst, accused of a number of murders - and even the subject of a national manhunt. He is currently on trial for the killing of Susan Berman.
Less Evil Roles: The Notebook, Crazy, Stupid, Love