Zooey Deschanel as Trillian from the film adaptation.
Tricia McMillan a.k.a. Trillian is a character from Douglas Adams' series The Hitchhiker's Guide to the Galaxy. According to the movie version, her middle name is Marie.
Trillian is a brilliant astrophysicist whom Arthur Dent completely failed to chat up at a party at a flat (possibly above an Estate Agent's) in Islington. The trilogy later reveals that Trillian eventually left the party with Zaphod Beeblebrox, who (according to the Quintessential Phase) is directly responsible for her nickname. She and Arthur come across each other but it isn't until the spaceship in deep space, six months after the aforementioned party and shortly after the Earth had been destroyed by the Vogons to make way for a hyperspace bypass.
Sandra Dickinson as Trillian from the TV adaptation.
In the radio series she is carried off and forcibly married to the President of the Algolian Chapter of the Galactic Rotary Club, and consequently does not appear in the second radio series at all. The later radio series (the Tertiary Phase and beyond) reveal this (probably) occurred only in the artificial universe within the Guide offices. In the books, which the third, fourth and fifth series follow, she saves the universe from the Krikketers, and later becomes a Sub-Etha Radio reporter under the name Trillian Astra.
The movie also hints at a relationship between Trillian and Arthur.
Trillian was played on radio by Susan Sheridan, on television by Sandra Dickinson (who also reprised an alternate-universe version of the role in the fifth and final radio series), on the Original Records LP version by Cindy Oswin, and in the recent film by Zooey Deschanel.
In the original radio series she is portrayed with a British accent — in both the TV series and movie she is played as an American. The Quintessential Phase of the radio series features Sandra Dickinson in the role of the alternate version of Tricia McMillan as a "blonder and more American" Trillian — the radio series indicates that the character is identical to the first Trillian and was born in the United Kingdom. In the book Mostly Harmless, it is said that both the alternate Tricia McMillan and Trillian have a British accent.
She appears in the books The Hitchhiker's Guide to the Galaxy; The Restaurant at the End of the Universe; Life, the Universe, and Everything; and Mostly Harmless. The alternate Tricia McMillan also makes a brief appearance in the radio series; Quintessential Phase and Quandary Phase.
Some drafts of the movie's screenplay, and Robbie Stamp's "making of" book covering the movie, state that Trillian was to be revealed as half-human. This would have been done in order to underline the loneliness of Arthur Dent, thus the only 100% Homo Sapiens remaining in the universe, after Earth's demolition. This idea was scrapped after the "making of" book was written, and the scene revealing Trillian's heritage (by the mice, to Arthur, on the Earth Mark II) was re-written.
Behind the scenes
The Trillian instant messaging software was named after this character.