In short: the Rik gives the words, the Sāman gives the breath. You not only understand it, you experience what you understand and live in it. That is sama veda.
The words do not change. The person saying them does.
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The role of the 4 vedas in yajnya.
1) Hotr - recites the Rigveda
2) Udgatr - sings the Samaveda
3) Adhvaryu - performs the rituals as per Yajurveda
4) Brahman - observes and supervises as per Atharvaveda