James Corden's latest Carpool Karaoke guest was the one and only Sam Smith, and it was definitely one of the funniest yet. Plus, the 'Late Late Show' host managed to make the singer's whole year by bringing along some very special guests and rendering him super starstruck.

Sam Smith at the Attitude AwardsSam Smith at the Attitude Awards

After first singing Sam's latest single 'Too Good at Goodbyes', he and James spoke about both their weight-loss journeys - one of which was a lot more successful than the other. However, Sam doesn't plan to stay slim forever by any means.

'I'm definitely down for getting fat again', he jokes. 'My plan is to keep my weight down for another ten years and then I just want to live in kaftans. When I'm older I want to be in a kaftan, no pants, glass of wine, huge [sunglasses]. And I just walk around with my Oscar... and go through photo albums.'

James and Sam also sang renditions of 'I'm Not The Only One', 'Stay With Me', 'Pray', 'Money On My Mind' and 'Lay Me Down' complete with some beautiful harmonies. Singing with Sam Smith has got to be James' biggest vocal challenge yet, but he nailed it.

Asked whether or not he ever gets starstruck, Sam confesses meeting actors is not always as he expects. 'I'm always confused when I meet actors', he said. 'They always look huge on screen and then you meet them and they're tiny.'

Nonetheless, his meeting with Ryan Gosling was definitely a stand-out moment (as it would be for anyone with eyes), but James Corden was about to go one better with their next song. Sam is a huge fan of girl band Fifth Harmony, and even wants to 'walk down the aisle to ['Work From Home'] in heels' after having a funeral for his 'single self' before he gets married.

Naturally, their karaoke version of 'Work From Home' would not be complete without Ally Brooke, Normani Kordei, Dinah Jane and Lauren Jauregui themselves and so James invites them into the back of his car for a singalong.

More: Sam Smith unveils new single 'Pray'

'Oh my God this is my dream!' Squealed a shaking Sam Smith, who is definitely inviting James along to his future wedding/funeral.