The 20-year-old actor - who plays Harry's sidekick Ron Weasley in the boy wizard films - was already anticipating a "big night" when he walked the red carpet in the pouring rain at London's Leicester Square on Tuesday (07.07.09).
He said: "It's great to be here. Premieres are always a lot of fun. Always a really big night - if you know what I mean!"
The next morning (08.07.09), Rupert seemed worse for wear, stumbling out of the exclusive Claridge's Mayfair hotel looking as though he might be sick at any moment.
The bleary-eyed star was so dazed he got into the wrong car, sitting there for around a minute before he realised his mistake.
The night before, Rupert bragged about his drunken antics at the premiere.
He laughed: "When I'm drunk I like to cause mischief. I'll steal props like the broomsticks from the 'Harry Potter' set and bring them out to the pub for comedy value. It seems funny at the time. Trouble is I usually get too trashed to remember what happened the next day."