The Spooks actress will play DI Georgina Dixon, sidekick to Toby Stephens's DI Jack Armstrong, in the BBC comedy drama's second series. She was brought in when Punch quit her role as DI Kate Bishop after the first edition of Vexed, which aired during 2010.
Speaking to Digital Spy about her experiences on Vexed, Raison recalled: "When I arrived the producer said to me 'rather than thinking in terms of the old character, it is a completely different and new character'.
"[Lucy] was absolutely brilliant... Lucy's bloody good, she's an amazingly natural comedienne. But [Georgina's] all brand new. I think what her and Jack have in common is that they both think they're really straightforward and they want to have successful relationships with people, but actually they self-sabotage all the time.
"That's a big difference between Lucy's character in the first series and Georgina. [Kate] was someone that had a life, whereas Georgina doesn't. She's quite sad. She's old enough that she should be further along in her personal life - not that she should try and be more conventional, just that she should be a bit more emotionally mature."
Raison went on to describe her Vexed alter ego as verging on "being completely unhinged" and teased that Georgina will have a "very different relationship" with Jack to Punch's character.
"There is definitely chemistry there," she stated. "It's not a Ross and Rachel [from Friends] where there's a great confession of love, but there's certainly quite strong chemistry.
"Georgina thinks she wants this conventional perfect man and Jack seems below the measure all along, but what she really wants is someone who is going to sit and watch football, because she is not cut out to be the other half in one of those relationships."
Vexed airs Wednesday (August 1) at 9pm on BBC Two.