The actress, who plays Grint's on-screen girlfriend Lavender Brown, admitted to BBC Radio 1 that she had been incredibly nervous about auditioning opposite the red-haired star.
Speaking on the station's 'Harry Potter Day', she said: "I built it up so much that when I turned up at the screen test I was so in awe of him. I was infatuated by him then, but I've moved on!
"Dan [Radcliffe] was quite funny when we did the kissing scene, jumping up and down in the background."
Grint (Ron Weasley) added: "There were eight different girls at the casting, it was quite a day. Jessie really stood out, she's quite a girl!"
Harry Potter And The Half-Blood Prince is in cinemas now.