Anne Dudek played Lassiter's partner, Lucinda Barry, in the pilot episode of Psych, but she was quickly replaced by Juliet (Maggie Lawson). Starting with Psych season 1, episode 2, Juliet was the main love interest of Shawn Spencer (James Roday), one of the show's two main protagonists, and was also partnered with Carlton Lassiter (Timothy Omundson) at the Santa Barbara police department. Juliet worked alongside Lassiter in solving homicide cases, with Shawn and Gus (Dulé Hill) always getting in the way and coming to their own conclusions about each death.
Before Juliet was on Psych, Lassiter had a completely different character as his partner. Played by Covert Affairs actress Anne Dudek, Junior Detective Lucinda Barry was the one working with Lassiter when Shawn and Gus first came onto the scene. Shawn was quick to deduce through his natural gift of observance that Lassiter - who was currently separated from his wife - was having an affair with Lucinda. Following this discovery, Anne Dudek's character was never seen again after Psych's pilot. At the beginning of episode 2, Shawn met Juliet in a diner, and learned that she was transferred to Santa Barbara to become Lassiter's new partner.
Lucinda Barry would be referenced a few times after Psych's pilot, but the show largely left her behind once Jules stepped in. Considering Juliet O'Hara's Psych introduction made her Shawn's series-long romantic interest, it's curious as to what the plan for Shawn's love life would have been had Anne Dudek stuck around. While the firing range scene between Shawn and Anne Dudek's character in Psych's pilot hinted at a possible flirtation, it paled in comparison to the spark between James Roday and Maggie Lawson. Nonetheless, Anne Dudek would remain a relic of Psych's past after the pilot, with only a few hints at what happened to Lucinda Barry after her departure.
Why Anne Dudek Left Psych
Behind the scenes, Psych's producers and writers decided not to keep Anne Dudek for the rest of the series because her pilot character didn't test well with audiences. Psych creator Steve Franks has explained that Lucinda Barry's affair with Lassiter made a negative impact on their opinion of Lassiter's character. It was mentioned briefly in the Psych pilot that Lassiter was separated, but not everyone in the test audience caught on to this, and some took it that Lassiter was cheating on his wife. Though Franks wanted Lassiter to be an "antagonist" to Shawn, they still wanted Psych viewers to like him. Franks has said that after moving on from Anne Dudek and Lucinda Barry, they cast Maggie Lawson, and that was "the magic of what made everything work" (via EW).
In hindsight, it's easy to see why replacing Anne Dudek's Lucinda Barry with Maggie Lawson's Juliet O'Hara was the best direction for the Psych to take after the pilot, as she brought a lot to the table in terms of her chemistry with Shawn, and the dynamic she developed with Timothy Omundson's Carlton Lassiter. Juliet's friendly, sensitive, and approachable personality made her the perfect contrast to Psych's Lassiter, who could be harsh, unfeeling, and aloof. The character of Juliet created a number of memorable moments through her interactions with many of the characters - particularly Shawn and Lassiter - and none of them would have been possible if Anne Dudek stayed in the role of Lassiter's partner after the Psych pilot.
What Happened To Lucinda Barry In Psych
The reason for Lucinda Barry's departure after Psych's pilot given within the show was her affair with Lassiter, which was, of course, against department protocol. Since Shawn had exposed them, Anne Dudek's Psych character had to be transferred. Juliet made a comment about this early in Psych season 1 by letting Lassiter know that their relationship would be different, and Lassie's affair would be joked about by the fake psychic Shawn later in the series. While this was never explained or explicitly mentioned, it seems that Lucinda Barry died sometime after Psych's pilot. In Psych's season 7 premiere episode "Santabarbaratown 2," Shawn and Gus go to the firing range and see that Lassiter has won an award for the "Lucinda Barry Memorial Tournament."