Emma was polite and acknowledged her, but she just wanted to be with Johnny and was clearly under his spell. They’re obviously head-over-heels in love.’
Johnny has spent the night with Emma at the upmarket house in Santa Monica where she has been staying with friends.
A source said: ‘The moment Johnny turned up Emma was all over him. He stayed overnight at least one night this week. It is clearly a serious relationship.
They might have denied it before but they don’t seem to be trying to hide it now.’
Another friend said: ‘Emma loves how down-to-earth and normal Johnny is. He’s very charming and sweet.’
Emma and Johnny were seen together in a park in June, but at the time her publicist strongly denied they were an item.
That month at the MTV Movie Awards afterparty Johnny was spotted pinching Emma’s bottom. It was only recently Emma lamented how hard it is to find a boyfriend.
She said: ‘I say to my friends, “Why hasn’t x called me?” They’re like, “Probably because they are intimidated.” It must be the fame wall.
I find it hard to believe anyone would find me intimidating.’
Johnny – Alabama-born and brought up in Texas – has a past in the tradition of Southern ‘good ol’ boys’.
He was arrested aged 19 in May 2006 for going at 80mph in a 65mph zone. Two days later he was arrested again for being in possession of alcohol below the legal age of 21 and was fined £350.
The 2009 film Jennifer’s Body featured him and Amanda Seyfried in a sex scene which he told his parents not to watch because he was so embarrassed.
He was lauded by critics for playing a drug addict in The Greatest, also starring Pierce Brosnan.
Emma and Johnny met on the set of The Perks Of Being A Wallflower in May.
Due for release next year, the movie is adapted from a controversial novel that couldn’t be further removed from Harry Potter, featuring drugs, sex and suicide.
Emma, who earned an estimated £19 million last year thanks to the final two Harry Potter movies and deals with Lancome and Burberry, recently announced she will attend Oxford University this autumn in an exchange year from her studies at Brown University in the US.
Her former loves include financier Jay Barrymore, eight years her senior, and musician and fellow Burberry model George Craig.
Spokesmen for Johnny and Emma did not return calls or comment last night.