Why do MP's stand up in parliament when the prime minister answers questions?

It may differ from country to country but for the UK then MPs stand at Prime Minister's Question Time in order to be considered (by the Speaker who runs the session) to ask a follow on question. They usually stand immediately before the PM gives an answer in order to register this request.

Take a look at this (short) video which is where the Speaker of the House himself is explaining the procedure: http://www.parliament.uk/about/podcasts/theworkofp...

It is mainly to keep things orderly and not lead to numerous cross conversations.

The PM is informed from his caucus on all of the matters at hand and together they plan the steps to follow so the whole Government agrees on their actions

Chetak