My sister told me that ‘celebs’ and the like are trained to smile as if they were taking a bite from an apple without allowing their lips to touch it, in order to give that full beam Hollywood smile and show off the glitzy teeth.
On the odd occasion I’ve tried it for photos, the results have not been quite as successful.
I now tend to go for the Posh Spice press-you-lips-together-and-flare-your-nostrils, as it’s easier to pull off quickly (with a little practice) and less likely to scare children.