'Excuse me, please don't block my view.'?:
To me, as a BrE speaker, I would say that is very polite and would work at home. You would be probably more likely to just hear or use; "Excuse me" on its own. Usually, the implication will be clear to the recipient because, they will turn round and notice that they are blocking your view. You have to say it positively and politely. (Usage A1:
http://dictionary.cambridge.org/dictionary/british/excuse). :up:
Don't say it with too much emphasis though, because that will have the opposite effect by making it slightly confrontational. (Second usage under A2:
http://dictionary.cambridge.org/dictionary/british/excuse) . :argue:
The phrase for use at the cinema is more difficult. In the past you probably could have also used, "Excuse me" there too, but people you don't know can interpret phrases in odd ways. Many people will probably have their own preferred phrases for this situation. Using, "'Excuse me, please don't block my view.'?", although polite may annoy the person you are speaking to, especially if they aren't really able to do anything about the fact they are blocking your view, for example if they are very tall.
If they can do something about it, I would probably just use, "
Please could you / stop bouncing in your seat / waving you hands in the air / moving your head from side to side.". (Believe me, I have seen some strange behaviour at cinemas!).