Sam Nicholls is a fictional character from the BBC medical drama Casualty, played by Charlotte Salt. She first appeared in the twenty-sixth series episode "Mea Culpa", broadcast on 15 October 2011. Sam is a former major in the Royal Army Medical Corps, who joined the Holby City Hospital Emergency Department on a nine-month secondment, later becoming a Specialty registrar in emergency medicine. Salt was contracted for a year and she spoke with army medics to help her prepare for the role.

Sam is portrayed as being a fearless, headstrong, competitive "action girl". Salt said Sam is also haunted by her time in Afghanistan. A few months after her arrival, it emerges that Sam is married to fellow ED doctor Dylan Keogh (William Beck). The estranged couple struggle to stay professional, especially when Sam files for divorce. A storyline arc focusing on whether Sam used excessive force on a patient began in early 2012. A formal complaint is made against Sam, resulting in a General Medical Council hearing. The character's backstory is also explored further in a special Red Button episode set in Afghanistan.

Sam was later given a love interest in Tom Kent (Oliver Coleman). The couple's relationship is tested by her friendship with Iain Dean (Michael Stevenson), but they later marry and make their last appearances in twenty-eighth series episode "Away in a Manger", which aired on 14 December 2013. On 25 April 2017, it was announced that Salt had reprised her role as Sam. Salt said she was "thrilled" to return and was excited about telling Sam's story once again. Sam returned on 23 September 2017, as a paramedic. During her second stint, Sam has a brief relationship with nurse Jacob Masters (Charles Venn). When Salt wanted to leave the show once more, producers decided to kill her character off, and Sam's final scenes aired in episode 1094, broadcast on 18 August 2018. Sam was well received by critics and viewers.

On 30 September 2011, the BBC announced actress Charlotte Salt had joined the cast of Casualty as "ambitious army medic" Sam Nicholls. Her casting was announced alongside that of Oliver Coleman, who joined as paediatric doctor Tom Kent. Series producer at the time, Nikki Wilson, commented that the character would be exciting and intriguing when she made her debut in October. Salt was initially contracted for a year, and she admitted that joining a long-running drama felt "quite scary" to her. To prepare for the role, she spoke with some army medics, including one who had worked on helicopters in Afghanistan. Salt also had to train for the various stunts that Sam was a part of, but she found the medical jargon to be the toughest aspect of the role. She stated, "It wasn't easy getting my head around it all at the beginning but it does get easier with practise!"

Headstrong and fearless, Sam is passionate about saving lives and not afraid to take risks. Before coming to Holby, Sam was an army medic who served in Afghanistan – where she thrived on the adrenaline rush of combat medicine. Sam is a brilliant mate, great to share a pint and banter with, but she's definitely not one to overshare.

— An excerpt from Sam's profile on the official Casualty website.

Ahead of her introduction, Sam was billed as being "fast-living, fearless and fiery" by the BBC. Salt later said Sam is "feisty, sure of herself and rubs people up the wrong way." She also described her as being competitive and someone who says what everyone else is thinking. Salt dubbed Sam "Casualty's answer to Lara Croft!" She told Patrick McLennan of What's on TV that Sam was a tough, "action girl", who was keen to save everyone, but she did not want her to seem like a "war machine". The actress also told McLennan that Sam was a fun character to portray, and hoped viewers would like her "energy and enthusiasm." Salt did not want Sam to be too likeable, as she thought that would be boring. Sam is a former major in the army, who comes to Holby on secondment, after serving in Afghanistan. Salt stated that Sam would find it hard to adapt to working at Holby, after being in a war zone, and that she would be haunted by her time over there. Salt also said, "Because the work is so traumatic you need a break. That's why my character is spending nine months with the NHS."

Sam turns up to work at the hospital a day early, and decides to go out with paramedics Dixie Dixon (Jane Hazlegrove) and Jeff Collier (Matt Bardock). They attend to Father Vincent McConnell (George Costigan), a priest who has been attacked by Paul Caffrey (Danny Sapani), as he believes the priest abused his brother. Salt told Katy Moon of Inside Soap that Sam approaches the scene "in true Army style" which annoys Dixie and Jeff. Sam manages to deal with an angry Paul, but he later uses his vehicle to ram the ambulance, causing it to overturn and injure Sam. Salt called Sam's first episode "fantastic" and "very exciting". She also explained that the situation showcases Sam's selfless side, as all she cares about is saving lives despite being injured herself. Salt said Sam manages to antagonise her colleagues, especially Nick Jordan (Michael French). She later added that Sam would clash with Zoe Hanna (Sunetra Sarker), as she does not get on well with women, having come from a job that was dominated by men. Salt enjoyed portraying the scenes between Sam and Zoe, and joked "I don't want to be nice ever!"