Can you smile on your passport photo?

Not in the UK. They'll reject it - also no thick or remotely tinted spectacles as this will interfere with the Iris Recognition (an idea that has yet to actually work at any of the airports I have been to).

When you get your passport photograph done you fill in a form at the same time. The form gives you distinct guidelines which tells you not to smile. If you chose to ignore these instructions I'm afraid your form will be returned and you would have wasted your money, and have the hassle of doing it all over again. Good luck with your form, it can be a nightmare filling it in correctly as so many get rejected. You can ask the post office to check your form before sending it off, but it will cost your about £10, but, it's worth it.

A slight smile but not to show open mouth or lots of teeth.

Yes, I don't see why you cannot smile. However, New Jersey recently announced that you cannot smile for your driver license. At this time, federal government hasn't say anything yet about smiling policy.

No - it says so in clear terms on the form.

Nope - your expression should be neutral and relaxed.

What actually identifies a person are such as the eyebrows, the forehead and the chin and so forth.

http://www.google.co.uk/imgres?q=victorian+mug+sho...

Nope you gotta have a complete no expressioned face

Yep

As long as it's not too noticeable you probably can

yeah