The short answer
Most countries want a neutral expression, with your face relaxed, mouth closed and eyes open. The safest choice for any country is a calm, neutral face. A big smile with teeth is the most common reason for a retake.
United States
The US State Department says you may use a neutral expression or a natural smile, with both eyes open. A very wide smile can still cause trouble, so keep it small and natural. Use the US passport photo maker to size the 2×2 inch photo.
United Kingdom
UK passport photos need a neutral expression with your mouth closed. Do not smile and do not show teeth. See the UK photo requirements and the UK photo maker.
Canada and Australia
Both countries ask for a neutral expression with a closed mouth. Check the Canada and Australia pages, and always confirm the current rules on the official government website.
How to look natural without smiling
Relax your jaw, breathe out before the shot, and think of something neutral. Look straight at the lens, keep your head level and avoid raising your eyebrows. Take several shots and choose the most relaxed one.
Why the rule exists
Face-matching systems at borders compare your face to the photo. A neutral expression gives them the most reliable comparison, so a smile or open mouth can reduce accuracy.
Before you submit
Check the head size, the plain background and the file size. If a portal has a limit, use the 200KB tool. See all passport photo requirements.