The character was portrayed by actress Marina Sirtis.
Marina Sirtis at first read for the role of Tasha Yar in 1986. She had, in total, five readings all with Gene Roddenberry and other executives. It has been noted that Roddenberry took a liking to her almost immediately. Denise Crosby, who eventually won the role of Tasha, auditioned for the role of Deanna Troi. It was said that Marina Sirtis had a more "exotic" feel about her (as stated in the dvd extras found on a bonus Star Trek TNG DVD).
She was just about to return home, in debt and jobless when she received "the phone call" alerting her to the fact that she had the role of Deanna Troi. She stated that had it been but an hour later she would have missed it and been on her way to England.
In the series, Deanna Troi served as ship's counselor aboard the Starfleet starship USS Enterprise (designated NCC-1701-D & NCC-1701-E and referred to as starships in the series). She served under the command of Captain Jean-Luc Picard (portrayed by actor Patrick Stewart).
Deanna Troi was born on March 29, 2336, near Lake El-Nar, Betazed. Deanna's parents are a Betazoid mother and Ambassador, Lwaxana Troi (portrayed by actress Majel Barrett, real life widow of television director/producer/writer and Star Trek creator Gene Roddenberry) and a human father, deceased Starfleet officer Lt. Ian Andrew Troi. In the character background, an older sister, Kestra, drowned during Deanna's infancy (note: "Dark Page"). Although Deanna Troi has little exposure to planet Earth culture, she attended Starfleet Academy from 2355 to 2359, as well as the University on Betazed, and earned an advanced degree in psychology.
In depicting a possible phenomenon of galactic travel, the series creates several fictional alien races including a Betazoid race that have telepathic abilities. Due to a half-human heritage, Deanna Troi has partial telepathic abilities and is an extra-sensory empath with clairsentience. As viewed in Star Trek: Nemesis, Troi has the ability to connect to another psychic and follow that empathic bond to its source. In this instance, her ability enables the Enterprise-E to target and hit the Romulan vessel Scimitar, despite the fact that it was cloaked. There are several species who are resistant to the telepathy of Deanna and other Betazoids; most notably are the Ferengi and the Breen.
Early in the series, Troi finds herself working with an early love, the newly assigned Executive Officer to the USS Enterprise Commander William T. Riker (a character portrayed by actor Jonathan Frakes). In later episodes, Troi has romantic involvements with several others, including a brief relationship with Klingon Starfleet officer Lt. Worf (portrayed by actor Michael Dorn).
She is called various names, usually depending upon the circumstances: Captain Picard calls her "Counselor", but when he is concerned about her or in emergencies, he called her by her first name: "Deanna." Doctor Crusher (one of her most noted female friends) usually calls her "Troi". Data very rarely calls her by her first name, but usually he calls her "Counselor Troi". Depending on the situation, Cmdr. Riker will call her: "Deanna" or the more personal "Imzadi" which means beloved.
Throughout Star Trek TNG, all primary characters have their own "Key" episodes. These include:
Season 1: Encounter at Farpoint - Start of her service onboard the USS Enterprise.Skin of Evil - Troi is trapped inside a shuttle leading to the resulting death of her colleague and friend.
Season 2: The Child - Deanna becomes pregnant to an alien host.
Season 4: The Loss - Deanna temporarily loses her empathic abilities and learns to depend upon her "human half".
Season 7: All Good Things - Deanna dies in an alternate future.
After Season 4, Deanna takes charge of the Enterprise-D, after it is seriously damaged by an unknown space entity. Initially hesitant, she grows into her first command role. By putting the safety of her crew before her own, she demonstrates her value as a leader.
In the 'alternate future' of the series finale, Deanna Troi dies tragically. Her death greatly upsets Riker, hinting towards their romantic involvement, following the continuity of Star Trek Nemesis. However, Captain Picard prevails, and the 'alternate future' is thus avoided, leaving Troi alive.
Finally, in 2379, (2002 film: Star Trek: Nemesis), Deanna Troi and William Riker marry. They both transfer to the USS Titan where Riker assumes command and Deanna Troi continues as an experienced counselor.
Phil Farrand, author of The Nitpicker's Guides, often complained about the way Troi was costumed and filmed. "Why does Troi get to wear the skin-tight bunny suit? ... Why would Troi want to wear the bunny suit? ... How would you react to a psychologist dressed like this?
It has been noted by fans that a collision between starships and other vessels on Star Trek is seen very rarely, but on two occasions viewers see Deanna crash both the USS Enterprise-D and USS Enterprise-E. She crashes the Enterprise D in Star Trek Generations after the fight with the Klingons and the destruction of the Enterprise's warpdrive. Commander Riker instructs her to "Take the helm" and for the first time, Deanna Troi pilots the ship and results in its crash on Veridian III.
Subsequently, in Star Trek Nemesis, Troi is once again commanded to "take the helm" by Captain Picard and he instructs her via computerised messaging to collide the Enterprise-E with the Reman vessel the Scimitar. She prevails by firing the ships thrusters and the impending collision caused massive damage to both vessels.
On many episodes, Deanna Troi falls victim to aliens. She is usually targeted by aliens when they take control of her mind and then talk through her body. A key episode in Season 4, entitled "Clues," is when the Enterprise's crew lose a day's memory. As events unfold, Deanna is taken over by an entity in order to communicate with the crew. In addition she also gains temporary "super-human" strength and is able to toss Worf across the bridge and effortlessly break his wrist. Sometime during/after season 5, the Enterprise encounters an alien species who are telepathic and specialize in being able to bring back lost memories. One of the aliens mentally assaults Deanna and also tries to physically assault her in her quarters. She is saved by Worf and one of his security teams. In addition, she is also mentally raped by Shinzon's Viceroy who is also telepathic on the film Star Trek Nemesis. The first time is in her quarters with her new husband and long time friend Commander Riker and the second time is seen on Star Trek Nemesis' bonus deleted scenes where she is mentally raped again in the turbolift (these deleted scenes are considered to be canon and are actual events of the film, but not included due to its length).