Jamie is the assistant director of the film in which Kat stars (in season 3).
Jamie and Kat do not get along, initially. He appears to find her frivolity irritating, and she seems to find him too uptight.
Shortly into filming, Kat essentially confesses that she is insecure, and wonders if she is good enough at acting and/or dancing to deserve the lead role she was given. Not long after Kat's confession, Jamie makes one of his own, admitting that Kat is not the only one feeling insecure. They form a connection, and become great friends throughout the rest of filming.
Before the wrap party, Jamie agrees to dance, if Kat will be his date. She laughs, until she realizes he is serious. Kat accepts.
At the wrap party, Jamie makes it clear he's got feelings for Kat. She replies that they make good mates (friends). Though once the blooper reel starts rolling, the director makes a comment about the camera loving Jamie. This leads Kat to realize that she does see more in Jamie than simply a mate.
When Ollie is performing on stage, Kat and Jamie get up and start dancing to break the ice for him, to lessen the awkwardness. Jamie dances badly, and Kat kisses him to get him to stop.