Emotion must be apparent on a speaker’s face. You don’t have to be declaiming on the steps of the Lincoln Memorial, or shouting in the House of Commons, but the principle is the same. If you want to move your audience, you yourself must be visibly moved, which means you must make your face expressive.
Of course any expression, frown or smile, can be monotonous in the eyes of your listeners. Your goal should be to smile, frown, grimace, or glower–whatever–and then wipe that expression off your face, moving on to the next.
Practice the subtleties of expression that mean so much–the sarcastic sidelong glance, the look of shocked incredulity, or amused condescension, a wistful smile, an angry thrust of the chin–the possibilities are endless.
Energize your performance. Modulate your voice. Animate your face. No matter how technical the topic, there is always the need for energy. Don’t be a stone face on Mt. Rushmore. Practice facial expressions. Vary the tone of your voice, raise the volume and pitch to emphasize important points, lowering it to convey intensity, even employing the stage whisper to draw special attention to your most powerful point.
Grand theatrics are not needed, but emotional involvement with what you are talking about is the key to keeping audience attention and convincing them that you are sincere.