How should an actor prepare to deliver a long speech?
As in all good acting, the answer to that question--like the answer to all questions seeking performance excellence--lies in the understanding of basic human behavior.
If an actor is required by a script to deliver a long speech, the actor should first ask: How...and why...do people in everyday life talk in an extended fashion? What is the psychological and emotional human dynamic behind a long speech?
For starters, let's accept that talking--even a word much less a long speech-- takes mental and physical energy. And...the human machine--like all entities in a long-evolving universe--is built for economy of function: it seeks to do as little as possible to achieve the desired goal.
So: people speak a long time not
because they want to but because they have
to; they go on and on because they can't convince the other person in a scene(or themselves in a monologue or soliloquy) in any shorter period of time. And believe me, human physics says they would if they could.
The reasons for extended speech are several: either the speaker can't sort through their own emotions cause by the prior event in an efficient fashion, or they can't sort through their own verbal facility to find the right words to express the depth of what they are feeling, or--and this is the most likely reason--the other person is resisting their verbal argument(s) so well...and thereby forcing the long-winded speaker to even become more long-winded.
An extended speech or monologue is one person (mono + logos, the Greek word for word) speaking, while the other person is spoken to; whether real or imaginary, whether offstage or on, whether self- or other, the other person is responding non-verbally. And the other character, the non-speaker, in spite of being perhaps offstage is refusing to be convinced! That is why the speaker of words has to continue a long harangue. A long speech is an extended 'dialogue'; that is, between two: one, the participant, the speaker-of-words, and the other, the perhaps non-verbal and antagonistic responder.
So: when an actor is preparing to speak a long speech, the actor must obey the logic of human behavior and prepare to speak one line at a time, each verbal utterance only in response to a non-verbal reaction of the adversary; each line motivated by the speaker's overall attempt to win the argument. The monologist offers each line of the long speech to be a 'convincer', the final (and winning) verbal attempt; each continuing dialogue of a speaker's long speech uttered only because of the unfortunate failure of the previous line to convince.